Question is whether they were compiling for the same architecture on both x86 and arm.

That's a good question but I don't think it would make a huge difference. Those details should have been included.

Safari is already a universal binary on my Intel Mac running Big Sur; that means WebKit runs natively on Intel and M1 processors.

It doesn’t, I’ve been compiling both for a while now and the amount of time it takes for each is almost the same.

Wow, the M1 MBP is on par with the 12-core Mac Pro from 2019 for the WebKit compilation. And even more impressive: "After a single build of WebKit, the M1 MacBook Pro had a massive 91% of its battery left. I tried multiple tests here and I could have easily run a full build of WebKit 8-9 times on one charge of the M1 MacBook’s battery. In comparison, I could have gotten through about 3 on the 16” and the 13” 2020 model only had one go in it."