That’s correct. Another way of putting it: The chain offered by the server is purely informational, and clients will build their own path to a trust anchor. Clients that have X2 in their trust store would potentially save an RSA verification, depending on what path they build.
Also, once X2 becomes ubiquitous (in a while), servers could offer the plain E1->leaf chain.
It’s true, this is weird and not consistent. Naming is hard, though, and this is what we’ve settled on for this round.
When we adopted X1 for our original root and X1-4 for our original intermediates, I think we were just following the example of DST Root X3; I don’t know the origin of that naming scheme.
For the new root, I’m following the example of our X1 root, which will live a long time, and not worrying about the naming scheme of the X1-4 intermediates, since they are not long for this world.
Our criteria were: under .com, .net, or .org; short; evocative of Let’s Encrypt. I’m pretty sure all the four-letter ones are taken. lencr seemed reasonably close in the five-letter range.