The important thing here is that while Intel is driving this, the code must be landed in browsers, which are not owned by Intel. Those browsers are not going to ship code that only works on one type of hardware, or is weirdly unoptimized on some hardware.
Having dealt with cpuid, there were plenty of caveats with how AMD did things vs how Intel did things. I can totally see Intel first checking to see if it was Intel and then punting just due to complexity and testing. Keep in mind that this article mentions p4 and athlon, so Intel would have also have had to care about Cyrix and Transmeta as well, which were different as well.
