I think you under estimate just what level expertise and pull modern web engine requires. Google funded a ton of the work to support clang building code for Windows explicitly so they could standardize on clang for Chromium on all platforms. In turn they used the fact that they use a single toolchain across all platforms to add support for things like control flow integrity, which involved low level ABI and runtime changes to to the compiler and libc++. That requires tight cooperation between the Chromium and the compiler teams, and is definitely beyond a typical "end-user of compilers and tooling."

Admittedly my claims about Chromium are based on my observations as an outsider, but as someone who works on tooling (linkers) at a different large company that also has a web engine written in C++ I can tell you it is consistent with what I have seen up close... web engines (and particular JavaScript JITs) tend to push C++ tooling hard and often inspire major investment by the teams working on C++ specifically to support them.

In fact, over the years I have seen a number of people (including C++ committee members) who transitioned back and forth between the C++ compiler teams and the JavaScript JIT teams because they sometimes work very closely together, so in my experience the level C++ expertise on lot of the teams developing web engines tends to exceed anything but the actual C++ tooling teams supporting them, and developing C++ tooling is most certainly in their wheelhouse.