Of the five largest tech companies only 1 has created a mono repo in git after Google and Facebook chose mercurial and started proving it out. I think it’s less about what the project was designed for and the communities ability to appreciate the challenges facing these large companies. You can tell from the post because all the feedback is “have you tried doing something totally different” rather than “that’s an interesting scaling challenge - how can we make git perform better at scale? what are the properties we’d have to trade off? how do we manage that tension or is this really fundamentally incompatible and if so clearly communicating why it’s incompatible with the project’s goals”.

At a technical level I believe both Google and Facebook engineers make core changes to mercurial when they need to so I don’t think that’s the root philosophical difference.