Sure, you could use tools that target the HDL as an IL, if you wanted to write that code yourself and then ship it to your customers as a binary blob, as their

Sure, you could use tools that target the HDL as an IL, if you wanted to write that code yourself and then ship it to your customers as a binary blob, as their HDL compilers need to be specifically licensed, and aren't, as far as I know, something designed to be used as a back-end component in shipping software. But that limits its usefulness for something like, say, writing a GLSL compiler so you can use your FPGA as a GPU, and are then able to run arbitrary GLSL code on it. I agree that it's definitely a much different than writing a compiler backend for a CPU. You are definitely going to have different abstractions that will apply well to it. But that's fine; that's what I'm interested in, writing a compiler for a very different kind of platform. The problem is, as far as I can tell, they don't document their actual bitstream format, so you must use their licensed tools any time you actually want to compile something new for the FPGA. And that precludes a lot of use cases.