Is this the reason why Nexon won't release the Secret crystals?

I don't think Secret Crystals had any purpose to begin with, they seemed to be placeholders for something else. Not lowering or extending level limit.

Secret Crystal has an item description that explicitly states they lower the level requirements.

Anyway the idea of using dark crystals to raise level requirements is a stupid assumption with no basis in fact since that's not even how dark crystals work anyway.