That seems very unlikely, what with fat and the lipids in cell walls - got a source?
* Energetic consequences of thermal and nonthermal food processing Carmody, R. N., Weintraub, G. S., Wrangham, R. W., Proceedings of the National Academy of Sciences2011 / 11 Vol. 108; Iss. 48
* https://booksc.org/book/37997813/2fc522
Note that the 'Atwater' system is referring to the traditional method used to determine caloric content created in the 1890s which burns food and measures the heat obtained from it.
Here is an interesting article about this issue:
* https://www.economist.com/1843/2019/02/28/death-of-the-calor...
I apologize for the single source -- I believed that the science was far more settled on this issue than it appears to be. Regardless, I hope it proves informative for further research if you decide to pursue it.
Uncooked meat does not have near as many digestible calories density wise as cooked grains. Of course it would be a lot harder for them to become overweight, but the food cost would be tremendous. Remember that obligate carnivores that weigh a lot, like tigers, are incredibly expensive to feed.