Churchill ALLIED with Stalin. Some things transcend hatred. Survival tends to be that thing.
That is exactly why I used this particular comparison - thank you for noticing!
Churchill allied with Stalin against Hitler. Gruumsh might become aware of an imminent threat from Nyarlathotep, and thus decide to ally with Corellon Larethian. If Gruumsh and Corellon have each other blocked on direct messaging, then Gruumsh might actively WANT a cleric who's in the same party as an elf, to establish a line of negotiation, on the basis of that cleric and that elf working together on a smaller scale.
My current main PC is paladin of Ancients and warlock with Archfey patron, in the same party as a warlock with Raven Queen patron. The two patrons have a shared enemy, a necromancer is an offense against life itself (according to the Archfey) and a threat to death itself (according to the Raven Queen). If those two can work together...
See also, Darmok and Jelad at Tanagra.