The SID chip uses subtractive synthesis, whereas this synth uses frequency modulation. They are very different and as such it's very hard to get the FM synth to sound close to the SID chip.
I wouldnt call the sid being subtractive synthesis, just because theres a multimode filter at the end, afterall it got hardsync and ringmod, which are additive.
I've only heard of additive referring to synthesis through summing series of sine waves. Ring mod and hard sync are pretty common in subtractive synths (my Minilogue has both for example).
Additive would generate sounds without any sort of time series "filter", and would attenuate the sine series before generating them.