My understanding is that chip QA at Intel took somewhat of a nosedive post Haswell. From my ignorant but interested outsider perspective, everything from Broadwell on seemed to be a mess execution-wise compared to Haswell (modulo TSX), and _especially_ compared to Ivy Bridge.

Some of the recorded comments on https://danluu.com/cpu-bugs/ (First update section) mesh with my observations, but I wouldn't know enough to tell if I was on to something, or just confirming my own biases.

Intel willingly admitting mistakes is just about as likely as hell freezing over

I'm inclined to believe that this is actually the case and not that AMD wrote more secure software. All software has security vulnerabilities, the more eyes on the software the more of them are found.