I don't agree with that.
Of course everyone has an own opinion and this is mine;
SG has 3 mainskills (JS, JR and DR) and 3 secondary skills (PA,OD and HD)
PG has 3 mainskills (PA,OD and HD) and 3 secondary skills (JS, JR and DR)
C has 3 mainskills (IS,ID and RB), 1 important sideskill (SB) and 2 secondary skills (JS, DR)
A point guard don't need to be a prominent shooter, he can still perform well without shooting skills, he will boost the defense and the offensive flow.
If you want to use a C as a PF then he would use way more of his JS and DR, since most teams are using C's as a PF they need decent skills there too.
Don't you think it should even out over time, all players have important skills and important other skills.
I didn't mention PA as an important skill of a C, my all star center got a nice passing level and he is in the toplist of assist per game. So yeah, the question is, what's important?
I do think it's too easy to say inside players need only 4 skills, and mainly guard trainers will say that