I did talk about this in part 1 of my episode review but since you actually sent this ask in before I’d even watched the episode I’ll just sum up what I said there for extra emphasis… (even though this has been meta’d to death by this point)
The show tells us via Metatron that Castiel’s true weakness is that he is in love with humanity. Metatron’s words. in love with humanity. Not “he loves..” or “he chooses…” no. the words used were “in love”. Which is only ever used in a romantic sense.
Ishim calls Lily his “human weakness”.
Ishim said he was “in love” with Lily. In Ishim’s mind, that love was true, romantic love.
Therefore we can conclude that Ishim’s “human weakness” was being in love with Lily.
Ishim then says that Castiel also has a human weakness. Like Metatron, Ishim also thinks Castiel is “in love”.
Ishim threatens to cut out Castiel’s human weakness. Saying that it will cure Castiel. He then goes to kill Dean.
Therefore, Ishim thinks that Castiel is in love with Dean.
This is like the mega upgrade to Metatron’s line. Metatron’s “He’s in love… with humanity” was followed only an episode later by the phrase “you draped yourself in the flag of heaven, but it was all about saving one human.” Which blew all our minds because this was Metatron’s way of saying that Castiel was in love with Dean. But it was still built into the subtext and spread across two episodes. It was still hidden basically, so to not be too obvious.
Ishim’s line though? There is nothing subtle about that. This is undeniable. I don’t see how the anti’s could possibly argue this. Ishim saw Dean and Castiel’s interaction, along with what he knew of Castiel’s history already, and came to his own conclusion, just like Metatron before him. That conclusion is solid and canon in the show. Both Metatron and Ishim thought that Castiel is in love with Dean Winchester.
So basically when people say that Destiel doesn’t exist, they are denying the canon of the show. Isn’t that a beautiful thing?