But after last chapter doesnt that pretty much confirms that its not? Endeavor didnt seem to recognize Dabi. And Dabi said “Nice to meet u” u obviously wouldnt say that to your father. So i think we can put that theory to bed.
Tho i do still think they have some sort of connection. The fact that the aurthor even put Dabi in front of Endeavor like this has to mean something. Especially since im sure he’s aware of the theories, At least to some degree.
Im guessing Endeavor failed to save him at some point. And it’ll probably be something like Dabi was a huge fan of Endeavor and wanted to be jus like him cause they had similar quirks
I mean if the author decides to make it happen, they still could and it wouldn't be full of plot holes most likely but yes you are correct it doesn't seem likely. I suppose he could still be related though.