That went right over my head...
It might well have done, but I'm sure you can understand the concept that what's claimed in the advert or web page doesn't translate to reality because the manufacturer is using smoke and mirrors, right? I mean that bit isn't too difficult to understand.
Incidentally, one of my commercial suppliers does a version of that in-wall amp. It doesn't have the fancy blue front but it's essentially the same guts inside. It is interesting to see their power rating because whatever they supply for commercial use does actually have to deliver on the promise made. Here's their figures:
Output Impedance: 4 Ohm
THD: 0.03% [my note: measured at very low distortion to ensure a usable power figure]
Frequency Response: 20Hz-20kHz [my note: measured with a signal frequency range that replicates real music]
Rated Power: 2 x 3w (RMS)
Their rating is 3W per channel. The amp from your link was rated at 25W per channel, but I said that 1/10th of that figure would be a more realistic rule of thumb number. 25W/10 = 2.5W and that's not far off what the same amp is rated at for commercial use.
Yes, you'd be safer with the Yamaha amp. The power rating of 2 x 85W is given with some reference to how it was measured: It's a minimum RMS power, and measured in to 8 Ohms using a full frequency test tone (20Hz-20kHz) and at very low distortion (0.019% THD) which all translates to a power figure that is deliverable in real terms
The only thing to add is to be aware that the KEF Soundlight system is a 4 Ohm load. That's a low Ohms number. So if your plan is still to add some garden speakers at some time in the future then try to make sure that they are at least a 6Ohm load, and preferably 8 Ohms. The reason is when driving both sets of speakers together (whether intentionally or by accident) then you really want to avoid giving the amp a hard time with two sets of low Ohms rated speakers.