This is a good point, and I have thought about it a lot before starting the book. What finally made my choose T3X is that its compiler is much smaller[1] than my smallest C-subset compiler and (IMHO) T3X is easier to learn or understand.
[1] SubC: 3815 lines, T3X/0: 2330 lines.
Of course you could start on CP/M without any outside tools, but then you would have to write your bootstrapping compiler in assembly language. Time-consuming, but certainly manageable. I doubt that it would be an interesting reading, though.
I'm -slightly- surprised it isn't using a C-like syntax rather than ALGOL-like but that's probably my own biases, and mentally mapping 'DO' and 'END' to '{' and '}' isn't much of a hardship.
Though if you'd care to share a link or something, I'm now curious how it -did- come to exist.