You are of course open source software existed for a long time before it was called that, way back to the early 1960's. People called it free software for a long time. And not just Stallman called it that, even the BSDs did and still do, despite hating RMS and all he stands for (GNU, GPL...)
ESR edits his texts a lot, and CATB came out before 1998. I don't know if I can track down a first edition of CATB and see what term ESR used here, but I bet it wasn't "open source". Note how he hyphenates it, according to his recommendation in the thing I linked.