So... you are, in fact, implying that there's merit in their attempt to claim this copyright?
If you think there's merit here, I'd love to know why. This seems like a slam dunk for Let's Encrypt under US trademark law.
Your point may be clear enough, but not why you think that way. I'm really curious about why a company would do this. It looks to me like a completely boneheaded move. If you explained, maybe that would give some insight.