I don't think they were implying that at all. I think they were making the accurate observation that she's definitely not going to get that from OP and their partner.

My ex-boyfriend, who is now one of my closest friends, grew up in foster care. The bad kind. The really bad kind. The "I spent a lot of nights holding him after he woke up screaming from the nightmares" kind. He was also eventually adopted by a foster mother he was placed with to get out of that situation; she has adopted 7 special needs foster kids to date and is still fostering and is one of the most incredible people I have ever met. There are a lot of foster parents out there who do it because they love kids or because they want to make the world a better place, but I am also under no illusions about how horrifically the foster system fails far too many of the kids who are in it. That does not make a couple who don't want kids, have no relationship with Lula, and had a bad relationship with her mother a good alternative, and the fact that they have a vague obligation to her doesn't change that fact. If we knew that a good home with someone else were an option, OP would be TA for following through on that obligation.

It may be the case that OP and their wife are the best alternative and if the question were "AITA for not taking in this kid I'm sort of obligated to knowing that if I don't she'll go into foster care?" that would be a much more complicated question, but it sounds to me like a lot of people who actually know Lula are trying to pawn the problem off on OP based on a technicality and foster care may not be the only other option if it's clear that OP won't let them.