You are actually arguing against his point. He said OEMs couldn't blame the carriers for slow update if they are slow with the updates in general. Your story proves that HTC did come out with updates early, which is why those custom ROMs are out there long before the carrier's version of the updates come out.
Nexus will always be the champion of fast update, but the other conclusion you should have taken from your experience is to not get the carrier version of the phone you want, but instead the unlocked version, which will get updates a lot faster. I made the mistake of buying an AT&T version of my M7 as well, and it was almost immediately got rooted so I could install custom ROMs, and then later on switched to the GPE version and after that all updates come from Google. But now with the A9 I bought the unlocked version and for the first time in almost six years of owning an Android phone I didn't root my phone, because I realized I didn't need it anymore, and part of it is that updates come reasonably fast and I have no need to install a custom ROM to get fast updates.