Not their fault, it's Lemmys pretty agressive code injection protection.
There were multiple attempts to let users put less than signs, but unfortunately each was found to be exploitable, so for now you just straight up cannot put a less than sign. There are also certain other things you can't type, even inside code rendering blocks. For example, you can't do C++ style include-statements. That's what I was trying to do when I learned all this.