Yes, LLVM has achieved the same contribution level as the Linux kernel[0], yet where are the contributions from ARM, Intel, IBM, Apple, Google, Green Hills, Codeplay (now Intel), NVidia, Nintendo, Sony, TI.... into clang?
While Apple and Google have switched focus to their own efforts (Swift, Objective-C, Carbon, C++17 being good enough), there are plenty of compiler vendors on that list with forked clang for their proprietary compilers.
And which of these proprietary Clang forks have greater C++20 compatibility than free software Clang?
The only proprietary fork of LLVM in the compatibility table you linked is less C++20 compliant than free software Clang.
So unless you have some causative explanation, I think the more sensible possibility is simply that the Clang developers have prioritized working on some of the plentiful other features of a compiler toolchain than perfecting their C++20 standards compliance.
Blindly asserting (or implying) that if LLVM were GPLv3 then it would be more standards‐compliant, with nothing to back it up, doesn’t add much to the discussion, IMO.
Who knows certainly? We would need to buy them them all.
Even if not, they are clearly waiting for others to do the needful for free, and then gladly recoup the fruits of others labour while selling their compilers for profit.
While Apple and Google have switched focus to their own efforts (Swift, Objective-C, Carbon, C++17 being good enough), there are plenty of compiler vendors on that list with forked clang for their proprietary compilers.
[0] - https://www.phoronix.com/news/LLVM-Record-Growth-2021