"Creo que ellos no se acuerdan de mí."
Translation:I think they don't remember me.
"Recordar" should work here. Duo used to use it more than "acordarse de". "Creo que ellos no me recuerdan" should be okay. Perhaps it is a forgotten answer.
I don't know for sure, but I think that the reason that "de mi" follows "se recuerdan" exactly because "recordar" is reflexive in this sentence. How else can you have the reflexive pronoun "se?" I've never heard anything like "Ellos no me se recuerdan," and I think this is incorrect.
Ellos no se acuerdan de.... means they don't remember.
Se acuerdan de... means to remember.
It's one of those weird reflexive se + verb conjugation + de combinations that changes meaning.