The definition of hacking on the Oxford English Dictionary: “gain unauthorized access to data in a system or computer.”
Technically speaking, this is hacking as you are accessing data that should never be accessed under normal circumstances. If you save in Pokemon, there is no way to go before the save, which makes the data before the save non-accessible. Since the data/gameplay is no longer accessible, you used the power save to gain access to that gameplay which is not accessible in a legit way. Therefore, you got unauthorized access of that gameplay. So yes, it is hacking.
However, my personal thoughts on hacking and cheating is that as long as it is done in single player and not brought into multiplayer, I don’t think it should be a problem for anyone else. People hack to make the story and Trainers of the story harder or to change Pokemon encounters. That doesn’t really bother me personally. But when people hack in a Magikarp with No Guard Fissure and battle others, they ruin other people’s experiences.
Again this is my personal outlook. People may have problems with you even when you hack on single and multiplayer. The outlook on hacking comes from moral and ethical reasons. Sorry for the lengthy response.
Sources: Oxford English Dictionary