> But report [1] says that "Some of these bugs showed evidence of memory corruption...", which implies that majority of these (which includes 271 bugs from Mythos) don't have evidence at all. Do I not understand something?

This is just the standard sentence we've been using for years. It has nothing to do with Mythos and for Mythos, almost all bugs show evidence of memory corruption (we do have a handful of bugs in JS IPC / JS Actors, one is in the blog post).

> Mythos is supposed to be pretty good at writing actual exploits, so (as I understand) there shouldn't be any serious problems with checking if bug is vulnerability or not.

Yes but if we have a choice between writing exploits and scanning more source, potentially finding more bugs, then of course we prioritize the latter.

> But report [1] says that "Some of these bugs showed evidence of memory corruption...", which implies that majority of these (which includes 271 bugs from Mythos) don't have evidence at all. Do I not understand something?

I'm guessing a bit, but for example: out of bounds reads are not memory corruption. Assertion failures in debug builds are also usually not memory corruption, and I'd guess that many of these bugs were found through assertions. (Some parts of Firefox like the SpiderMonkey JS engine make heavy use of assertions, and that's the biggest signal used for defect validation. An assertion firing is almost always treated as a real and serious problem. Though with our harness, Opus and Mythos try to come up with an exploit PoC anyway.)