That's (half the processing) true if they're downsampling from the whole sensor, but if they're not (e.g. R5 crop), then I'm not sure it still holds....A 12-24mp sensor requires doing ~half the information processing as a 36-61 mp sensor, and therefore generates half the heat. I have no doubt future models will bump that up but today it's not a flat comparison if evaluating the camera as a hybrid. If you're more video focused, those cameras will probably give you better results than the R5, and 24mp is a better compromise between video and stills.
Also, I've not seen anything that equates twice the amount of data to twice the amount of heat - it depends on how you process the data (line skip, downsample etc). What I don't know is the relative impact of the downsample vs encode. The 24HQ encode times seem to infer the HQ has significant processing.
But overall, if that were the case, then the would that not mean the R6 should have longer recording times before it overheats compared to the R5?