Given that Level 60 is the cap for composing, I would say that ALL 31 skills are capped at their (current) maximum levels ? - not sure where you get the 9 from 31 or 4 from 31 from ?
And who is to say that increasing the level of composing wouldn't, potentially, increase the level of some of the skills ? - I'm pretty sure that raising the cap from 50 to 60 did - although I'm more than happy to be corrected there.
9 form 31 is pretty straightforward, Pardoux. Just look at the wiki page for composing and you will see that 22 of the 31 potions are the same potion-level for composing-level 59 and 60, i.e. they have already reached their cap. There is no reason to expect these individual caps to change were the composing level increased.
Of the 9, 4 have reached roundish numbers at level 60 (500, 300, 250), so it is not unreasonable for Wolf to ponder whether they have now reached their cap.
Hence Wolf's conclusion: at most only 9 potions, and possibly as few as 5, would be affected by increasing the composing-level cap.