I don't think that's quite true, for several reasons:
* The biggest external contributors (Opera, Samsung, and Intel) put in more development effort than Pale Moon has, by a wide margin. (I think non-Google contributors to Chromium outnumber those working on Opera's Presto at its peak, though less than Opera's desktop product and Presto at that time.)
* Last time we saw any browser vendor leave the browser engine business (i.e., Opera killing Presto), we saw plenty of people jumping ship to keep working on browser engines (and not just those let go), so if we were hypothetically in a Chromium monoculture it's unlikely that Google pulling out would stop every Googler currently contributing from continuing to do so.
Also, when it comes to the Linux kernel, remember that both Windows and FreeBSD implement the Linux kernel syscall ABI too, which you can argue means they are alternative implementations of the kernel.
You could argue the Python/Linux case are safer because:
* They are themselves independent open source projects. While Chromium is open source, if Google pulled out, I'd expect the remaining development effort to be on par with e.g. Pale Moon
* You could argue there _are_ multiple implementations of Linux-as-OS, if you're not just looking at the kernel. See Ubuntu, Red Hat, Arch, Alpine, etc. There's no standards body (unless you count stuff like POSIX or LSB, but then they are only subsets of functionality compared to what users expect of a full OS) but still things become standard by community consensus (e.g. systemd)