While there are indeed big challenges to making the Martian atmosphere thick like the Earth's, I wouldn't buy the statement "since Mars has 38% of Earth’s gravity, it can only retain an atmosphere of about 0.38 bar".

Compare: "since Venus has 90% of Earth’s gravity, it can only retain an atmosphere of about 0.90 bar".

The relevant thing is the gas escape rate. Aside from the magnetic field, this is dependent upon the temperature, atmospheric composition and the escape velocity. Without plate tectonics to replenish it, even Earth would eventually lose much of its atmosphere. Note that Venus has a weak magnetic field also, and the interaction with the solar wind is believed to be one reason why there is so little water in the Venusian atmosphere.

So, if you were to add enough gas to the Martian atmosphere, it would have a higher pressure until the gas was lost over thousands or millions of years.

I wouldn't try aiming for terraforming Mars with today's technology, but that doesn't mean that tomorrow's technology might not be able to. Personally, I think Venus is the better choice for terraforming anyways.

It sounds wrong to me. Venus has about 100 times more atmosphere than the Earth with about the same gravity. Titan, which is smaller than Mars and half its gravity has an atmospheric pressure of 1.5 bar, higher than the Earth.

It is not the same composition, obviously, but I don't really see how it matters. On Venus, the atmosphere is mostly CO2, on Titan, mostly nitrogen, the gases we had on Earth before life came in and dumped all that oxygen.

So I don't see why a 1 bar, Earth-like atmosphere would be impossible on Mars, except for the small detail of getting the gases there in the first place.

It sounds odd to me since Venus has a surface pressure of 93 bar and a surface gravity of 0.904 g. My guess that a lot of other factors are at play (like the magnetic fields) that are more relevant than gravity. (My understanding of atmospheres is also very basic)

I think he meant that if mars had the same atmosphere in mass as in earth the pressure would be only 0.38bar. But if it had a more massive atmosphere there, pressure could rise more.

Hardly. We have serious problems with climate change that need to be addressed, and serious problems degrading the environment of other species. But outside of the most extreme nuclear winters we cannot make the planet inhospitable to life.

In his Aurora novel, he offhandly disowned his Mars trilogy by saying Mars was uncolonizable because of toxicity if it's soil (see calcium perchlorate), something he probably didn't know when he wrote the trilogy