"They will never FLY, but they could easily tear loose at the base and spill the air. "

The Zubrin quote is:
"...dome experiences an upward force of some 7000 tonnes..."

I said, "1 cm plastic dome weighs 13000 kgs"

Is that weight along the bottom rim of the dome going to lessen at all because you add pressure to the dome?

The problem is it's pressure vessel of which one side of this vessel is the ground.

If you had a hemisphere with a flat base, rigid enough remain flat [not become deformed] under whatever pressure you using is there any need of 10 meter deep foundation?

So, what is needed is to address the floor.

You should have a floor of some kind, but if you want only martian dirt floor, you could use a steel hoop 50 meters in diameter:

You put hoop on the ground and ensure that it is sealed so no air [at pressure you going to use] can pass between it and the ground. You don't need to worry about the hoop having upward lift. [Though it could have upward lift if air escapes between it and the ground- or Martian winds might cause this. Or explosions inside the dome. Or someone runs truck into a wall. Etc.]

Attach and seal dome to hoop.

Done.

[You don't need this steel hoop, but you need to make sure air doesn't escape between the dome and the ground- and some sort of hoop could make this easier. A hoop in a concrete footing might also be fairly easy.
{How you make and transport the dome is a completely different matter.}]

"THAT was Zubrin's point, that you need to actually hold DOWN a greenhouse roof on Mars."

Well, do you need to hold "the roof down" on any pressure vessel on Earth?

A roof of a pressure vessel *could* support weigh on it.

Say, you had some oil drum, which withstand 10 psi on sides and bottom but could only hold 5 psi on the top- if you pile up enough dirt the top of it, it should then be able withstand 10 psi- if 5 or more lbs per sq inch of dirt could be stacked on it. [At soil density of 2 grams per centimeter you need over 2 meters height on Earth and about 7 meters on Mars]

Or put 10 psi capacity oil drum in 33 feet of water- now, it can be pressured to 14.7 psi + 10 psi. At 330' feet: 147 psi + 10 psi- [and pressurize it only 10 psi and it easily gets crushed].

So, one could avoid making any spherical shapes and handle the pressure by simply having a flat roof with enough weight and strength- though this is daunting if one
requires sea level pressure on Earth.
On Earth we have a "roof" of about the mass 30' worth of a gas in liquid state. And on Mars it would require about 90'.
Obviously, using a pressure vessel seems and in many ways is much easier.

It's been suggested by others that on Earth we could cities made concrete and glass which could float in our atmosphere.
On Mars you could similar structures, but they aren't going to float in Mars' atmosphere.