One of the things to keep in mind about the Surrender Oaths is that they are only binding on the Yozi themselves. Nothing prevents Ligier from leaving Malfeas except the spite of Malfeas and Cecelyne. The escape clauses most demons get is not a flaw in the Oaths, it’s a flaw in the Yozi that allows their souls to slip free for a period of time.
If the Exalted decided to start murdering Yozi again, nothing prevents every single demon from attacking Creation en masse. Even during the High First Age this was considered a bad thing. Further, nothing prevents the Yozi from doing something like creating an unstoppable Creation-devouring monster that would destroy everything or engaging in Three Spheres Catalysm-level mass destruction. In fact, part of the Surrender Oaths was specifically addressing the threat that things like the Three Spheres Cataclysm may happen again; if they do so, the Yozi can be forced to tank the shot as well thus basically turning the Oaths into MAD.
The ability to summon demons was, in fact, a side effect of the oaths of surrender that bind the Yozi to answer the Titan Summoning Clarion. The Yozi let demons be summoned as a fragment of the oaths as part of one of their long-term escape plans. Of course, the Oaths were actually written by the Exalted and the Incarna so they foresaw a lot of this stuff and left the seeming loopholes in on purpose specifically so the Yozi would waste time on dead ends.
So if a Fetish soul snaps back to Malfeas upon being threatened by a spirit killer, it’s not the Oaths that do it but the Yozi itself going “Nope!”