TLDR: Hound coming back is a huge spoiler, not comparable to Stoneheart as showrunners stated she was not coming back. Cleganebowl is not a spoiler its a theory NOW that the episode is out. You can't use information implied from the book and tell us it might happen in the show when the show says nothing about the hound between his death and this episode. Saying Cleganebowl will happen was a spoiler because you're using information in the books that heavily implies the Hounds return and then forming a theory around it....
Its not a fan theory, its a THEORY. Fan theory would be me saying than Euron will rule simply because he wants 1000 ships, theres no evidence that supports this hypothesis. The fact that the gravedigger thing literally happened just shows how much evidence from the book lead to this conclusion. Ofcourse i have a hindsight bias but Kyle and Murkah made it seem like what happened in the books pretty much showed this was gonna happen.
As a show watcher I would have had no idea the hound was coming back, although I could sense it happening because Woody brought up cleganebowl and then in last weeks episode Cersei and Jamie hinted at a trial by combat with the Mountain. So you can argue that Woody spoiled his return for me. Its more annoying that Woody was arguing that it was the same as R+L=J. Cleganebowl theory comes from what is implied in the books that the hound is still alive, and thus suggesting the hound could come back in the TV show IS a spoiler if you give that information away. The argument that the show is ahead of the books so it isn't a spoiler is a logical fallacy because you're assuming that what is implied in the books cant happen in the TV show simply because some storylines are ahead.
R+L=J is different both because nobody who watches Game of Thones doesn't know this theory, and two because its been hinted at in the show so many times now and if anybody argues with me on that point then they should pay more attention. First time i remember it being referred to in an in your face way was when Stannis first spoke to Davos about Jon. Woody did have a strong point that Kyle brings up things like Tyrion being a Targaryen and I haven't read the books so i wouldn't know if thats a substantial theory that im gonna be pissed about if it happens.
BUT having said that, the hound coming back is a massive spoiler coming from somebody who has read the book -> to a show watcher like myself, although cleganebowl is only a theory. If woody had waited until the hounds return on the show then it wouldn't be too much of a spoiler because its then a logical theory from the information we have.
The only way talking about the hound in the context of the books wouldn't be a spoiler is if we literally saw the hound dead in the show. Sorry