I couldn't find it explicitly mentioned myself, but that is the logical assumption. No doubt Digital Foundry at least would've mentioned it if RT mode caps the framerate at 30fps in races, as that would be perhaps the most significant downgrade to the GT experience.
With how heavily implied the presence of engine swaps were, and because no one has found it elsewhere, tucking it in the roulette system is an interesting possibility. Maybe a way for PD to increase player retention since it's obvious that people want it. "Want engine swaps? Play more and rack up those roulettes" sort of thing.
I haven't kept up with any streams or reviews (outside of Jordan's and Digital Foundry's), so I have no idea if the tuning menu/GT Auto options paint a clearer picture as to whether or not this could be the case.