It’s separate from Amanita if you split it from Amanita; it’s part of Amanita if you don’t.
The fact that a clade is monophyletic is no point in favor of recognizing it as it’s own genus. Every species is monophyletic.
See the recent huge Cui et al paper on the Amanitaceae (which maintains a split Limacella) for a reasoned scholarly rejection of Saproamanita.
If MO is ever contradicting all publishing mycologists, there might be a problem, but that is far from the case here.
MO doesn’t recognize Saproamanita