Now that I have a ton of time to spare due to the virus quarantine, I made some tests to prove my point. And because I did not want to ruin my opera installation, I installed opera beta, on v68 today, and did all the testing there.
So, 10 days after my original question, here is some proof that the libffmpeg from other electron based project works too. The libffmpeg in question comes from skypeforlinux. I just downloaded the deb file, extracted libffmpeg from it and moved it to opera's folder. It's the same procedure that the thread and the "wiki" on reddit describes, but with libffmpeg coming a different source.
Without it, the usual poor, codec support, with most notable the lack of h264 decoding
And with it, that... precious h264 support is now available and all sites work as they should.
So the questoin remains. Why should someone prefer that libffmpeg from the github repo and not from some other electron app?
In my eyes, both give the user an equal amount of risk to break their system or the browser.