If the Peter principle is just “you succeed until you don’t” then it’s tautological and applies in all contexts.

no, it's not the Peter Principle.

The Peter Principle is pointing out that if you keep getting promoted by doing an exceptional job, and you stop getting promoted when you stop, that you have ended up leaving a job you were exceptional at, and landing in a job you are not good at. In a certain sense it's saying "you should be demoted back to your last job that you were exceptional at"; it's not suggesting that, it's suggesting a sort of snarky "this whole system is set up to fail" because it's from a "isn't capitalism grubby" perspective.

None of this flavor is in the general YC scaling case, though you may be able to find it in various places within that.

I think you'd be hard-pressed to make a case that VC-motivated business scaling does not consistently overinflate the businesses which are artificially propped up by investor cash. There are a bunch of companies, existing and dead, that show the folly of growth-at-high-cost.

that's not the Peter Principle