Hold up there son, and take that twist out'a ya pants.

If Mars had had a 200 mbar atmosphere as recently as 100 Ma, then we would know from the condition of the surface.

I've described before the rules of thumb about meteorites reaching the surface and the craters that they generate. To repeat, for those whomissed the earlier episode:-

1) Meteors disintegrate once they have collided with their own mass of planetary/atmospheric material. The atmosphere of earth corresponds to a 10m layer of water (4m of rock), so an impactor needs to be a minimum of 4m before it can reach the surface.

2) Meteors blast a crater ~20 times their diameter, so the minimum crater diameter on Earth is ~80m. (clusters and secondaries are different - these can be smaller)

3) We can scale to Mars. Today, a 6 mbar atmosphere is equivalent to 18cm of dry ice, so the minimum impactor is ~7cm. Min. crater size 1.4 m (no micrometeorite pitting on Mars)

4) We can scale to a hypothetical early mars. 200 mbar on Mars is actually 52% of the integrated column mass of Earth's atmosphere (remember the scaling of pressure with g), so one would have a minimum impactor size of 2m, and a minimum crater size of 40m.

So, if your hypothesis were true, most of the surface of Mars would be devoid of 40 m and smaller craters. MOC images show this to be completely false.

If you posit that ALL the sub-40 m craters occurred in the past 100 Ma, you're into Grondine's territory of a rain of meteors so fierce that you would wear a tin hat to go outdoors.

AND, where is the fresh giant crater that displaced the atmosphere so recently? There isn't a distinctive, large, recent crater one can point to.

In fact, there are lots and lots of big impacts all through the history of Mars. If any one of these would disperse the atmosphere, they ALL would, so you would lose the atmosphere much earlier, which returns us to the status quo ante, IMHO...