There's more parts dissipating heat than the GPU/CPU. PS5 has a power hungry faster SSD and a few extra fixed function units.
I considered that before floating my thought, but the XsX is using the same Nvidia RTX IO solution in the XsX and that is PCIe, so it is either 10watts or 25watts for their 2.5GB/s - which is probably less power efficient that the PS5 SSD because Xbox made noise about cooling and performance when SSDs get hot from high throughput, so that still wouldn't offset the difference in PSU wattage IMHO., and the SSD in each is probably using less than 20watts IMO.
The TDP of the XsX chip was something Microsoft didn't want people to know, as they deflected the hotchips' question - with a excuse akin to suggesting it was too complicated to explain, despite the audience. The additional units in the PS5 CPU side of the APU will increase the TDP, but IMO it will be small wattages in the 5-10 watts difference on 7nm.
Ultimately GPU silicon is by far the most power hungry item in both consoles, and with the PS5 APU having just 70% of the CUs of the XsX, and XsX GPU clock being 82% of the PS5's maximum clock, the power needed for work-done in each system will have been selected by their engineer teams based on real-world capability - the XsX will have been done the way Cerny explained how the PS4 Pro power/cooling was designed in the Road to PS5.
Xbox have played the "fixed clocks" line, which if sincere should mean that at comparative max throughput as the PS5, it should probably have a 450watt PSU, because the PS5 power usage is a paradigm shift - of deterministic power, as Cerny describes removing the unknowns, so they aren't doing guess work - and so don't need the PSU headroom they needed in their other consoles (or the Xbox designs).
On that basis, the PS5 is will be using proportionally more of its PSU power than the PS4 Pro, and deterministically using more all the time, because of the constant boosting, and need to be transforming it into actual work-done to fit the new paradigm for cooling.
IMO, even if we accounted for some of the differences in external power output between the XsX and PS5 - the PS5 looks to have a USB3.0 type A hub(10watts max?) on the rear and a USB-type C hub on the front(36watts max?), and the internal NVMe upgrade can require 25watts like PCIe.
If by comparison the XsX only offers up 40-50watts of that 70watts - by not doing the NVMe option or supplying the 36watts for USB type-c, then if the actual internal XsX hardware was more power hungry for work-done then I'd still expect those other 16 CUs(14 CUs if we say Tempest is 2CUs) to push the XsX APU TDP by 40-50watts more. But seemingly it doesn't, and will have the same PSU headroom as the Pro or X1X designs did.
So, I think this is the biggest indicator, that despite the XsX fixed clocks the CU occupancy at those clocks will be low enough for that system to use less power than the less TFlops ps5.
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