Thermal output is where all the watts you pump into the CPU come back out. You can't have more thermal output from less wattage input or vice versa.

I believe the multi core performance claim in that the base M3 has 4+4 cores while the X Elite has 12. After that everything falls in line as "a much faster Qualcomm offering than before that's still a bit behind the competition". If it had released 2 CPU generations ago when they started talking about how fast it was it would have been more impressive.

I also believe the battery life claim in that if you put the laptop in airplane mode, then the screen all the way down, and run a video on loop it is fully possible it does this longer than Apple depending on how efficient the video decoder and ability to turn the rest off are. When it comes to actually using the laptop it'll be interesting what third party benchmarks say.

Nobody is measuring "total thermal output". That would dumb, because like you said that just corresponds 1:1 to battery usage. They're measuring how hot the device gets and how good it is at moving heat away from the critical parts of the device.