Agreed than he can't be a Dumbledore. The timeline speaks against this in every way.

The situation with the Obscurus yet - unfortunately, it's near impossible that it's Ariana's.

Credence developed his parasite due to the abuse by Mary Lou - and Grindelwald can't have brought it to the US and them (if he had managed to steal it/if an Obscurus doesn't die along with the body unless an expert like Newt manages to seperate/ nurture it in time), or he wouldn't have known the Obscurial wasn't Modesty - besides, she was about half Credence' age, and only was adopted once old enough to remember and talk about her siblings (as we hear), so I'd say 2 - 5 years ago tops - and since Credence is magic and, given the timeline, was forced by Mary Lou at least 20 years tops before to surpress his magic, it firmly must be his own.

(I believe he lived only because 1. he's indeed remarkably strong and 2. he had another adoptive sister, Chastity, who was a little older - we don't know much about their relationship, but I once read, though she was more loyal to the cause, her and Modesty often took punishments in his place ... I believe both their love, much like likley in Ariana's case with Aberforth and her mother, were the reason he lived on, despite the existence of his Obscurus which couldn't be helped)

(from what I puzzled together, once it exists, it can only be healed by being removed and a true brother or sister being there)

However ... I think him being an Obscurial may indeed have something to do with what Grindelwald told him ... it's either because, knowing Dumbledore and his past losses, he knows precisely why latter is trying to find the Obscurial, maybe even intends to personally try to heal him, as a way to deal with not having been able to help his sister ... or, going into the same direction, he knows Dumbledore is practically without a family by this point, but in their place, builds a network of friends who are lonely in similar ways and tries to be a sort of trusted/brother to them ... and he knows Credence would be given the same attention and support if they met.