Nah it's not. Who knows? It's still pretty early. Maybe it depends on how things go?
Yeah, Anderson seems very busy and fulfilled with her current work and I don't know about Duchovny. But I think that if the event series does really well (which might or might not be a stretch depending on how much its recent pop-cultural awareness has improved due to the X-Files Files, Netflix, etc.), Fox will join Chris Carter is pushing for more X-Files. And maybe the limited season size will enable and encourage the stars (and the writers' imaginations)?
Hopefully Fox will leverage what they have going here and release a full-series Bluray set at Christmas time to prepare for January.
Another related thought: kind of disappointing, but if the show does well but A&D are not interested in returning, maybe Fox will have Chris Carter do another spinoff? I can only seriously wonder how well that would do and what quality it would be (though I loved "the Lone Gunmen" show, the Doggett/Reyes years just weren't the same).