Or you could implement Rust in C++.
As you say, you can do whatever you want in C++. The only question is how much time and effort it would take and whether it's worth it.
I will note that the people who worked on the Firefox style system had plenty of experience working on complex, high-profile C++ projects. Every single one of them that I've talked to agreed that for the specific thing they were doing here Rust allowed much faster development of code with fewer bugs (especially as measured by how many issues the fuzzers found) than their past experience with C++ had been.
If your point is that the claim should be "parallelizing the style system in C++ was impossible within the schedule and manpower constraints Firefox was operating under" rather than the simpler "was impossible" claim, then sure, as a purely logical-statement matter. But in practice there are always schedule and manpower constraints.
Can I prove mathematically that some other team would not have been able to achieve the same results in the same amount of time with C++? No, I can't; such a proof would be quite difficult to construct. I do have the empirical observation that there used to be at least four fairly different modern non-parallel CSS implementations out there written in C++ (Gecko, WebKit, Blink, Edge), and that one of them has disappeared (Edge), one was parallelized in Rust, and the remaining two remain in C++ and non-parallel, even though people generally agree that parallelizing them would be good and so forth.
Maybe it's just that none of the organizations involved are any good at writing C++ code. Maybe it's that doing this in C++ is hard enough that it's not worth the effort. Maybe it's something else. What is your hypothesis on the matter?