So what is a valid place/situation for the "both sides may be energized" label?
I can think of several examples where the label would or could be appropriate...
Definitely appropriate:
1) In an older-school off-grid system where PV panels are directly paralleled with a battery (and PV charge controller passes voltage through when energized, then an open disconnect for either PV or battery is energized on both sides when only one source is disconnected.
2) In a utility scale PV system where, say, you had multiple DC combiner boxes connected in parallel to, say, a common bus on a central inverter, then a disconnect at one combiner box will be energized on both sides unless all the combiners are disconnected from the common bus.
3) Where an AC output energy storage system with backup capability is connected to the grid but still outputs voltage when disconnected, the AC disconnect should have this label. e.g. Powerwall or Enphase Encharge.
Borderline appropriate:
4) There might be some inverters where capacitor discharge takes a while, meaning that a DC disconnect might still have some live voltage on the inverter side for some seconds after the DC disco is opened. Maybe this applies to your case here? But seems very, very persnickety if it's not longer than it takes to pick up a screwdriver.
5) Where you have AC and DC coming into an inverter enclosure, as in your case, I have found it somewhat appropriate to apply the label as a kind of 'don't touch this if you don't know what you're doing' warning. The idea being that someone unqualified may think that turning off the switch de-energizes one of the sets of conductors entering the enclosure, as would be true for many enclosures, but not in this case. But this is more of an extra-diligence idea than something that is code required. And another label such as 'dual power sources' could serve the same purpose.
As far as the 'crying wolf' applications I mentioned earlier, I mostly was referring to AC disconnects or panelboards where interactive inverters were connected. So many inspectors just don't have a clue about UL 1741 and normal inverter anti-islanding.