It really isn't though. PCC just walked the AST, writing out text to a file with hardly any optimizations just like most of what people call 'transpilers'. But you wouldn't call a c compiler to asm a transpiler, right? Pretty much everything that gets attributed to some intrinsic difference between source-source and source-machine compilers is just a function of the immature tooling on the web.
What I would do is ask 'what part there is any different than any other compiler?'
Why do you think it is helpful to be unambiguous between atoms that make up a human or atoms of any goods in the back of a truck? /s
> translates between programming languages that operate at approximately the same level of abstraction
That's fairly straightforward, though a bit subjective (which doesn't preclude a word from having meaning)
There's the (albeit not 100% correct) meme that C is portable PDP-11 asm. What is correct in my mind is that PCC has much fewer, much less complicated transformations to go to PDP-11 (or M68k) asm than Babel does to go from ES-next to ES5.
But for some reason Babel is a transpiler because it's all high level and that's magically different. And no one in their right mind would attempt to call the c compiler of the 1980s a transpiler.