I can promise you that fusing a moontide is possible. I just got it yeasterday and I didn't pay attention that well to the mono magic legendary I used (might have been harvester). Frankly, I didn't think that it would work.
When I first read that moontide was fusable again I fused lavithan and eatheral and got a black frost. Then I fused boiler plate (got from beast chest) and a mono legendary magic (I got a couple from beast chest, and I have a lot of mono legendarys from epic bosses I never fused for obvious reasons) and got moontide. 0 reasons to lie, it's a f***ing cell phone game. Bottom line: it works