The problem with many concepts in amateur radio is they are not always clearly defined, e.g. Centre of Activity frequency vs. Calling Frequency are two such concepts that had me arguing recently with Brian @G8ADD and others as to their meaning and usage [You’ve heard the joke about asking 10 economists to explain something and you get 11 different answers, well, ask 10 radio hams …. you know the rest].
Re COA frequencies for QRP, I was a strict QRP operator for over twenty years (I’ve lightened up a bit since then). My understanding of QRP COAs was to have a small part of the spectrum where all operators stick to QRP [assuming everyone cares to follow band plans]. The point is to see what’s possible with low power levels and simple equipment. It makes a mockery of the challenge if many of the operators are QRO. It’s expected they turn off the linear and wind the rig power down to 5W CW/10W SSB.
Regarding SOTA and QRP COAs, I usually operate at least a few kHz above or below the QRP COA as I know many chasers will be operating QRO, and in any case, if I take the KX2 instead of the FT817 I like to melt chasers front-end FETs with my blistering 10W of CW.