Because Fellay wishes for an agreement with Anti-Catholic rome.
Therefore, Fellay is against those who reject the Antipope.
He appears to be backtracking on his dealings with the modernists. Maybe he might end up being a sedevacantist himself.
No, he isn't. Do not be fooled by his recent words. They sound nice, but it's nothing more than damage control. He hasn't issued one apology for his attempt to sell his flock to the modernists and he never will apologize either. The "Superior General is always right" mentality is very prominent in the Neo-SSPX. It's cultish. If you disagree with it's SG (+Fellay), you're disciplined or given the boot. Archbishop Lefebvre wasn't like that.
How do you know this?
His insistence that the Resisters are "practical sedevacantists" is one reason. He and the Society are also telling people that Archbishop Lefebvre was hostile to sedevacantism, which is a lie.