"Waterfall" is not a development process. But plenty of the processes that give space for planning, also give space for QA. So surely agile, in the broad sense of development processes, is relevant to the comparison.
A development process is not either Agile or “waterfall” anyway.
By all "software engineering" books' definition, Waterfall is a software/development process. Good or bad...
Which book describes "waterfall" as an actual process, as opposed to something generally slow and bad?
Being an actual process and being slow and bad are not opposites of each other. Literally, every recent software engineering book has a section on the "Waterfall Process". Nobody claims waterfall is good, but it is a software development process.