So in general yes, if you don’t have copyright assignment it is a lot of work to relicense. But that is not necessarily the case here... things like the MIT license (which ziglang uses) are often so permissive that you can relicense them as GPL.
With that said this sort of thing can result in mass contributor exoduses. One might prefer an in-between license like a per-file copyleft as you see in the Mozilla Public License, which is still GPL-compatible but does not put the full onus of the GPL on the software, solving this problem in a more narrow way (connectFree has to keep their proprietary software separate from ziglang when combining the two together, which forces them to be much more transparent about their value-add—things which they cannot keep separate need to be contributed back upstream).
With that said this sort of thing can result in mass contributor exoduses. One might prefer an in-between license like a per-file copyleft as you see in the Mozilla Public License, which is still GPL-compatible but does not put the full onus of the GPL on the software, solving this problem in a more narrow way (connectFree has to keep their proprietary software separate from ziglang when combining the two together, which forces them to be much more transparent about their value-add—things which they cannot keep separate need to be contributed back upstream).