I wouldn't say they have less thock. Probably has more do with the plate and less play when key reaches the top.
Hmm, I've always heard that HHKB > FC660C > RF in terms of thock. You'd disagree? (I value your opinion a lot since you own all three)
I'd agree with that in terms of the volume of the thock. If you're talking about which one has the best thock, well I think that is more of a subjective nature. I think that the "thock" of these keyboards are all slightly different because of their unique construction, materials, and form factor.
When talking about the characteristics of thock, most people tend to prefer the raw thock of the HHKB because of the lack of a metal plate, therefore giving the maximum amount of sound. The RF 55g has a more deeper/low-tone and snappier/poppier sound due to the more responsive rubber cup and the addition of a metal plate, which I like equally to the HHKB. The 660C can pretty much be described as being in between the HHKB and the RF 55g, taking the best aspects of both boards and balancing out (lighter than 55g, soft bottoming out because of plate while still providing snappy and responsive feel with enough a good thock). This however, I've found, makes it kind of unexciting and I found myself typing on the 2 extremes (HHKB or 55g), but don't get me wrong, the 660C is an awesome all round board. In my opinion, the RF 45g is in between the HHKB and the 660C, which makes it kind of the combination of the downsides of the 2 boards, making it bland and boring.
I know I've said this before but I'll try to make a video of the HHKB and RF 55g, and maybe the RF 45g (23UB) so you can compare it to the sound of my 660C. I know they won't be too accurate but at least you'll have a comparison between all of them from the same environment and same source. You can adjust/calibrate based on the actual sound of your own 660C.