I think the distinction is meaningful - for example many compilers used to have C backends (GCC for example did) - so you code went through almost the entire compiler pipeline - from frontend to IR to backend where the backend did almost everything a compiler does, it only skipped target machine specific stuff like register allocation (possibly even that was done), arch specific optimizations and assembly generation.

A transpiler to me focuses on having to change or understand the code as little as possible - perhaps it can operate on the syntax level without having to understand scopes, variable types, the workings of the language. It does AST->AST transforms (or something even less sophisticated, like string manipulation).

In my mind, you could have a C++ to C transpiler (which removes C++ constructs and turns them into C ones, although C++ is impossible to compile without a rich understanding of the code), and you could have a C++ to C compiler, which would be a fully featured compiler, architected in the way I described in the start of the post, and these would be two entirely different pieces of software.

So I'd say the term is meaningful, even if not strictly well defined.