Ah, ok. But how so? You can get a LE cert as long as you can serve a file in the correct URL, or set a certain DNS record. I don't see why proxying would prevent that.
Oh, of course. I was thinking of Let's Encrypt's DNS-based authentication since that's the only thing I use nowadays (though of course Github isn't using that). Ignore me.
But yes as I said it doesn't matter for a blog in practice.