Haaaaang on, there
just might be a wee giveaway here about just how thick a "tenuous" atmosphere can be...
Wildly speculative hats on...
If you zoom right in to this piece of concept art, to the top left monitor above the window, there's what appears to be the status readout for an airlock.
It includes the text "Target ATM: 755 MMHG".
755 MMHG is about 1 Atmosphere. So what you'd expect for the interior pressure of a surface base.
But it also includes two percentage indicators. One at 71% and one at 21%.
It's impossible to say for sure, as the icons for these values are not immediately meaningful to me... maybe they're just filler the artist plopped in to pad out the display... or that the values themselves are plucked out of thin air for this concept... or this is just an early mockup and the in-game values have changed since this stage of development...
But it seems (maybe?) implicit that 71 could be the percentage of "Target ATM" currently measured inside the airlock, as it climbs towards the ideal 755 MMHG...
And 21% would be the baseline, ambient, exterior pressure
So if that (very reaching) assumption is correct, it might mean atmospheres up to about one quarter the thickness (or at least pressure) of Earth's.