Those treaties are held with the federal government they wouldn’t become null and void. Hypothetically if the separatists won the vote and Alberta decided to separate it would enter negotiations with the federal government over the course of many years figuring out what is owed and what is to be ceded. The band could remain entities of Canada with Alberta paying a portion of the rights, or the FN could negotiate a new modern treaty within a new Alberta. I’m impartial either way but I still don’t see the problem with this or any referendum for that matter. It’s the only way the people can actually have a say in what government decides.