Well I don't believe any of it. Show me the mathematical proof that it happened exactly that time ago, though we both know you can't. All of that is just statistics based on unproven assumptions. No one knows whether the Moon and Mars had similar impacts ar similar times or not, that's just another assumption. Most of the northern hemisphere on Mars has much fewer craters and recently it was found a lot are buried, so we don't know how many others are buried and where.
The whole thing is innately ridiculous that we can tell when one impact happened on another planet. We don't know where the rock came from, whether it was an asteroid or passing through the solar system, it's all just a mish mash of guesses. We don't even know from looking in the crater because there are so many ice signs that lots of craters could have impacted on the ice and the record could be wrong. Sprenke and Baker charted a polar wander that could have had a pole right in Hellas and it could have been there for a billion years for all anyone knows. You might say that not everyone accepts polar wander but that just proves my point, that there is no certainty about any of these things. If there was a pole there then all the crater records in Hellas would be changed and the basin would appear differently in age from erosion.
Also there's no way to date the crater from its erosion because no one knows how much of a previous atmosphere there was and how long it lasted. For example:

"Alba Patera: One of the largest shield volcanoes found on
the martian surface, the Alba Patera regional center of
magmatic-driven tectonism, is located in the northern
region of the Tharsis magmatic complex. Alba displays
many of the characteristics of Syria Planum, including
concentric and radial fault systems of varying relative
ages, pit crater chains, and lava flow fields. Distinctions
include: (1) magmatic-driven activity from at least the
Early Hesperian extending into the Amazonian [1,7-8],
(2) a prominent shield volcano marked by summit
calderas [e.g., 7] central to the concentric and radial fault
systems (perhaps marking a central vent), (3) a paucity of
shield fields, and (4) wrinkle ridges that transect the
summit [7]."

http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1811.pdf

This talks about early Hesperian not Noachian, and Alba Patera would have been formed from the Hellas impact. So there really is no censensus about the age of Hellas at all, this would have to be 3.6 billion years old too. The early Hesperian dates from 3 billion years so the so called exact 3.6 billion is wrong. So the argument then comes down to whether it was late or early Hesperian, and Alba Patera runs into the Amazonian.

http://cmex.coginst.uwf.edu/CMEX/data/SiteCat/sitecat2/geochron.htm

According to this the ages are known only to within a factor of two:

Adapting the lunar production curve to Mars requires knowledge of the difference in impact rates between the planetary bodies; the nature of crater-forming projectiles; and the effects of differing gravity, impact velocity, and target properties on impact crater formation (Strom et al., 1992; Ivanov, 2001). Modeling of these parameters permits an estimate of the ratio of the lunar to Martian production functions, allowing estimation of Martian crater ages to within about a factor of 2 (Ivanov, 2001; Hartmann and Neukum, 2001). The factor of 2 uncertainty in age has relatively little effect on interpretation of the absolute age of Noachian terrains, expected to have been originally nearly saturated with craters. Similarly, the factor of 2 has relatively little effect in interpretation of the age of very young terrains, where a surface with a nominal age of ~10 million years (Myr) is young in any case. However, the factor of 2 uncertainty means that ages of terrains which fall in middle Martian history are very poorly constrained. Model crater ages can constrain the boundaries between Mars’s major epochs as defined by its three time-stratigraphic systems. Compilation of various crater age estimates from the literature (Head et al., 2001) places the Noachian/Hesperian boundary in the age range 3.5 to 3.8 Gyr, and the Hesperian/Amazonian boundary within the broad age range of 1.5 to 3.5 Gyr. (A Gyr is 109 years.) A recent assessment by Hartmann and Neukum (2001) places the Noachian/Hesperian boundary in the nominal age range 3.5 to 3.7 Gyr, and the Hesperian/Amazonian boundary within the nominal range 2.9 to 3.3 Gyr. The uncertainty in the Martian crater production function implies that late Hesperian through mid-Amazonian ages are the most poorly known.

Also the times are based on assumptions:

Martian landscapes can be dated to three periods of its history, called Noachian, Hesperian and Amazonian. The absolute ages of these periods are very poorly known, and estimates depend on what assumptions are made about cratering rates. The Noachian is the oldest, and is older than 3 500 to 4 300 million years. This is followed by the Hesperian, older than 1 800 to 3 550, and the Amazonian is the youngest.

So depending on who you believe they could be a billion or more out in the age of Hellas, or as one said, a factor of two. To repeat "However, the factor of 2 uncertainty means
that ages of terrains which fall in middle Martian history are very poorly constrained."