Indeed, if those Sheliak are known for something, it's them being prepared for the least likely things through shelf-lightminutes of preemptively written legal jargon. That they'd make a treaty might be taken as solid proof that there never was conflict with them to spur such a treaty, then - instead, our heroes seem to believe that an actual conflict with the Sheliak would not be war but one-sided slaughter, and they clearly aren't veterans of that.
The "incident" thing apparently continues to rely on certain ill-founded assumptions:
1) That there'd be one, in what DSC eventually turned out to be about - that train might already have sailed to the sunset with Fuller.
2) That the incident affecting the 2250s would have taken place anytime close to the 2250s.
3) That the incident really was mentioned in TOS specifically, rather than being something merely relating to TOS and described ambiguously by Fuller.
4) That the incident was an incident marking a spot on the timeline and not, say, a character trait or a species description or whatever.
All those assumptions might still hold true, though. But as long as we don't know, we can't even tell if we already found out all about this incident in the first hour and half of DSC.
Timo Saloniemi