So I stand somewhat corrected, I had a memory of Chowning describing it as additive in his book "FM Theory & Applications" which I got with my TX11 but as it turns out he felt calling it additive was too simplistic as modulating a frequency is multiplicative. Each 'Op' however is added together to give the resulting tone. Now contrast that to my Arrick where I'll feed a square wave into a VCLF and peel the push filter out the harmonics over time to get a nice 'gong' sound which is definitely subtractive.

I did get a chance though to re-read chapter Chapter 3 "What is FM" and marvel at the relatively simple way it composes into very complex waveforms.