What's commonly left out here is that faster films, generally, have larger halide grains. This is why, for instance, Tri-X has always had a longer development time (same developer and temperature) than Plus-X.

And this same factor means faster films also take longer to fix -- a larger halide grain takes longer to dissolve, from the surface inward. This means that larger grains will have more time to develop in a monobath before the fixing action removes the halide.

Monobaths (especially modern ones based on a rapid fixer formula) use very fast-acting developers: fast enough that there would be risk of uneven development in a tank process (the usual rule of thumb is to make three minutes your minimum). This is offset by the rapid fixing, however, so uneven development isn't a problem with monobaths -- the fixing starts at the same time as development, and if this time varies as the tank fills, it doesn't matter because each square millimeter of film will start both processes together, and fixing regulates development.

In the end, what makes modern monobaths work equally well with, say, Ilford Pan F and Kodak T-Max P3200 is that the longer it takes to develop, the longer it'll take to fix.