I have seen the personalized-by-ship Spirit jerseys available in the gift shop on every RCI cruise I've taken. Enchantment, Grandeur, Explorer, Adventure, Freedom, Oasis, Anthem, Harmony, and Allure. My wife has four; I have one from Anthem. Enchantment is a lot to spell on the back!
My experience is that they have always had crew necks.
I have seen the knock-offs occasionally, but the Spirits were in the shop also.
I forget if it's a perk of Diamond (it is for D+), but there is a 20% discount for purchases over a certain amount. The Spirit jerseys were definitely over that amount.
They are the real deal and wear like iron.