This is FUD. The reason it's not possible to load microcode or other proprietary blobs from linux-libre is because of a limitation of the deblobbing process. From [0]:
Indeed, I became aware that some users have got the idea that blocking the loading of blobs is a feature. It's not; it's just a bug that's quite difficult to fix. The decision on whether or not to use a piece of software, be it Free or not, should belong to the users, and it's not our intent to make that difficult.
If you can make the deblobbing script leave an escape hatch for users to load their own blobs, at their option, I'm sure the pull request would be well-received.
[0] https://www.fsfla.org/ikiwiki/blogs/lxo/2013-11-08-linux-lib...