They are sufficient for pro licenses, though, which are what they mention as the required baseline.

I was looking at a quarter million for a few dozen machines and 2 AD servers and 4 SQL servers. Nearly on par with the cost of the hardware I was buying. Until some bright spark realized it was all testing/dev/qa for those machines, MSDN suddenly was way better cost wise. If all you are doing is testing/dev then MSDN is usually the choice you want (but grab your lawyer to dig into the terms on that one). But in a real env with no dev work, the licenses become a real pain and very costly, and per year recurring many times, and stack because each service type has time, or sometimes they combine. Not dealing with it at all is very tempting.

There's no requirement to purchase an enterprise license. Large companies just do that to save money.

Was your need to maintain KMS actually caused by a Microsoft requirement, or was it rather a cost saving and standarization measure?

Microsoft licensing goes way beyond Windows - there‘s the whole range of office software (which you can also license on Mac), then all the server software, etc., most of which should does not apply to Gitlab (unless they want to deploy on linux and switch to SQL Server as database, but I doubt that). And then, if you‘re big enough, you can negotiate corporate licensing to save money, which requires license management etc. I doubt any of this applies to Gitlab as they stand.