HF designs can be isolated. The switching transformers provide the isolation in the same way that the output transformer in a LF design does. That doesn't mean a given HF inverter uses transformers. It used to be possible to get boost HVDC stages, ie used inductors instead of transformers and yes, people did die. As we've seen in other threads manufacturers do dumb things with wiring too breaking the AC / DC isolation.
AFAIK MPP uses transformers in the HDVC stage but beyond that someone would need to do some testing if MPP doesn't produce any certifications etc.
Although it shouldn't need to be said I'm talking about isolation between the battery / DC / PV side and the AC output. I don't know of any country that requires isolation AC to AC because that makes it impossible for a grid tied inverter to exist since... it's the same wires! Even high voltage non-grid-tied PV (ie what the MPP is) systems use some form of voltage adjustment ie HF transformer (or potentially buck/boost <shudder>) to bring array voltage to DC suitable to PWM generate the AC output.
