Does that mean he will have decent primaries for a PG but his primary skills for big man are likley to be lower?
Basically yes. And by lower they could be none existent ... you never know.
That might be ok if his potential is ok as I can train up the skills he needs fast.
Or is it a waste for a huge guy listed as a PG?
If you want a well rounded player then it is awesome, but as you pointed out he would need superstar + potential to be worth it given his primaries are likely starting from behind where you really would want them to be. If he has the potential then you could end up with a super player, just be prepared for it to take time though.