Where did he say this? In any case, I can confirm it from personal experience. x86 isn't that annoying if you ignore most of it. The worst thing is the edge cases with how some instructions can only use some registers, like shifting/rotating using cl.

There was a video of him talking about it, but I can't recall the exact words or the name of the video.

But his operating system, TempleOS, was based on HolyC which was compiled to pure x64, and had support for inline x64 assembly.