> were they just run the program in their heads?

This is itself a skill people need to learn, that I'm not sure is possible with pseudocode and no prior experience. Too easy to gloss over details without actually running it to learn where your blind spots are.

I did this workshop a decade or so ago where I learned my co-workers don't do this, and never did learn how they understand code otherwise. One of them mentioned he didn't even realize this was a thing.