"This is not always true,"
No? So then tell me which intel Mac has seen longer support by macOS than by Windows?
"and you forget the driver story with Windows/PC. Also things like BIOS and EFI security updates. The majority of these "supported" Intel processors as you put it do not have BIOS fixes for eg for the spectre bug and variants. Only the extent of protections offered by the OS SW based mitigations exist."
That is not true. Pretty much everything starting from Sandy Bridge and later has seen fixes for these issues, and BIOS/UEFI fixes have been quickly made available by the big PC vendors such as HP and Dell. Of course, the latter may not be the case for any junk system out there but that's simply the result of buying, well, junk which its manufacturer has no interest supporting.
Besides, it's not that the same CPU bugs didn't affect Macs. For example, the Nehalem/Westmere based Mac Pros are as vulnerable to the bugs in its processors as any other Nehalem/Westmere system. However, while Apple has stopped support for these computers after Mohave (i.e. three macOS generations ago), they happily run the latest version of Windows 10. Which contains workarounds for at least some of the bugs in its processors.
And then there are the unfixable bugs in Apple's own hardware, like the early T2 security processors in some Macs which have an unfixable critical security hole:
"When Apple say supported, it is the whole thing from boot firmware, drivers, os, hw, service support."
Sure, just that Apple may or may not actually fix a problem. And I'm not just talking about the many instances where Apple denied responsibility for problems caused by idiotic design decision (like butterfly keyboards or too short display cables, or spontaneously cracking displays on its early M1 Macbooks). Because unless you're always on the latest macOS version then you may have to wait a long time to get patches for a security problem that was quickly fixed in the latest version of macOS:
Then there are the various minor issues which remain unfixed by Apple, like the issues M1 Macs have with many standard monitors.
That's Apple "support" for you.
"Put another way, any SW not written by Microsoft is almost certainly vulnerable on older PCs."
The same can be said about a Mac, which normally runs 3rd party applications.
But as shown above, even if you're only using Apple software you may well be left out in the cold waiting for a fix for a critical update.
"Also as for Microsoft, support is dropped but masked by the use of "it is running Windows 10", which is the latest right?
What you miss here is that "support" in Apple's circles has a different meaning than "support" does for Microsoft. Because just because Apple "supports" a specific Mac model or macOS version doesn't mean it's actually providing fixes for problems that are discovered on these products. For Microsoft, "supported" means that it will actually go and fix problems that occur on a supported platform.
Also, because Apple actively prevents the installation of new macOS versions on unsupported Macs, having a Mac that's "supported" is a lot more important on the Mac side than it is on the PC side. Even Windows 11 installs fine on older PCs (the oldest one I have it running is from 2012, and that's only because I have no PC that is older to try) without any hacks.