No, the debate is about gun caliber.
TLDR: HEAP, essentially, gives a net improvement to HE shells fired from guns that are of smaller caliber than 155mm (with the extent of the net gain varying between ships and tiers). Those can be main or secondary guns, as you see fit. But the gun needs to fire HE in the first place.
So, it doesn't really make sense to use them in T7-8 IJN BBs, because those ships mostly fire AP from their secondaries. So, you're using a 4-point skill for no actual gain. And it doesn't make any sense to use HEAP on the massive main guns of those ships anyway.