Given that C99 is much closer in vintage to their current C89 than it is to today's date and also just straight up really old, it does seem inappropriate to distinguish that as "modern C".
That said they do seem to be discussing both C99 and C11 so it does seem like going with something that would qualify as modern (especially in light of Linux's need to be conservative) is actually on the table.
You also have to look at the contents of the revisions. C99 was a massive update for C, and C11 was tiny in comparison (and C17 was basically nothing at all).
Writing C89, without variable declaration in a for loop, and without // comments, etc, feels ancient. C99 on the other hand is what defines how C looks today.
This is also why the attitude is: if they jump to c99, they may as well jump to c11 because they are basically the same.
That said they do seem to be discussing both C99 and C11 so it does seem like going with something that would qualify as modern (especially in light of Linux's need to be conservative) is actually on the table.