"Fully" smells like a weasel word here. Either it is E2EE or it isn't. iMesssage isn't by default from what you are saying and if it requires the other end to also turn off icloud backup before it is E2EE then I'd go as far as stating that it is a completely useless attempt to be E2EE. In fact I'd argue Apple is full of sh*t if they actually ever stared that it is E2EE (but I have no idea if they did).
Comparing Signal to such a mess is... well at a minimum it is disingenuous.