Of course I'm aware of this, I've written some assembly too, most definitions are leaky. And if GNU assembly had wide adoption among programmers right now and an ecosystem around it, then some people might also call GCC a transpiler (in that specific mode, which is not the default), if they care about the fact that it outputs in a language that they may read or write by hand comfortably.
They also called C a high-level language at that time. There was also more emphasis on the distinction between assemblers and compilers. Indeed, they may have used the word compiler more in the sense we use transpiler now, I'm sure people were also saying that it was just a fancy assembler. Terminology shifts.
I think what happened was that, when writing in assembly language was a common thing to do, programmers had a clearer idea of what a compiler did, so they knew better than to say "transpiler".
They also called C a high-level language at that time. There was also more emphasis on the distinction between assemblers and compilers. Indeed, they may have used the word compiler more in the sense we use transpiler now, I'm sure people were also saying that it was just a fancy assembler. Terminology shifts.