They have not been formally condemned as a group to be heretics, so no. The people making up the NO are at worst material heretics.
Now, if you know individuals who are formal heretics, by all means avoid praying with them.
This.
This is the logical conclusion, though, from Sedevacantists. They have deposed the pope, the hierarchy, and the whole Church. It's a fine line perhaps, but a line one must not cross nonetheless.
If you are saying the Pope is a formal heretic, the Conciliar Church is not the Catholic Church "full stop", they are all heretics "full stop", then you wonder about stuff like this -- should I associate with them? Can I pray with them? etc.
If you ask me, it's another argument against Sedevacantism. A
reductio ad absurdam if you will.
And I should point out: most Sedevacantists are also Hierarchy-vacantist and Ecclesiavacantist. It is NEVER just the Pope they condemn and completely reject as 100% heretical.