The Knights are a religious order in the only way that matters: the leaders are drawn from "professed knights", who profess the vows of poverty, chastity and obedience. The pope doesn't need any more authority than that to deal with them.  
You may know more about the Knights than I do; you may be right in thinking that their vows don't mean much to them. While a future pope could reverse the actions of this one, if the Knights choose to violate, ignore or renounce their vows, it would be a steep climb for a future pope to rehabilitate them. If their identity as a medieval military order is in conflict with their religious vows, or if their military identity is more important to them than their religious identity and vows, then by all means they ought to be dispensed from their vows.
<Someone in the Vatican, it seems, has jumped the gun here.> The investigation will determine who that was. My money’s on Cardinal Burke, who has been the proverbial ‘bull in a china shop’ for forty years.