The main point of the debate about open only power play is the concept of fair play.
Let's say you're in the middle of playing a chess game with someone. Then say there's a knock at the door and you have to get up to go answer it. Whatever it is is distracting enough to completely absorb your attention while you're dealing with it. Now while you're at the door, your opponent moves a couple of their pieces to put themselves in a better position. When you return from the door, you reacquaint yourself with the board and maybe have a feeling that something is off, maybe you just play on, but the game takes much longer or perhaps you lose because of the changes made while you were away from the board.
Now with power play as it currently is, when you and your opponent are playing in open, you can see each other's pieces, you have a way to counter whatever they might do provided you have the skill / knowledge to do so.
When your opponent does things like undermining or expansion in solo, it's like the opponent moving the chess pieces while you're at the door. You have no way of knowing what's going on, you have no way to counter what they're doing, the game is no longer fair.
It's the unfair and unbalanced nature of this which causes the argument to be relevant and why those of us who force ourselves to play fair and intentionally avoid exploits, afk / scripted grinding and stay in open tend to feel so strongly about the argument.
Now ideally the best solution for this is for FDev to design PP 2.0 so that it closes those holes, that the system is fair and balanced, that there is no action that does not have a method for countering regardless of what mode a player is in. If a mode requires players to face each other in order to support fair play, then it should require players to be in open. If it is counterable through a mechanism and people's efforts regardless of mode is represented in the UI in a visible and plain way that clearly shows that countering is necessary, then restricting it to open isn't necessary.
As power play currently is, the only way to counter some actions is to directly intervene with the player. If it's impossible to do that, then the game is not fair. In case someone has it in mind that NPCs are the counterbalance, let's be honest, outside of the Thargoids, NPCs are not a significant threat to anyone who's been playing the game for any significant amount of time. NPCs are not sufficient countermeasure, they are endorphin generaters to let the player feel competent and successful as they are erased while the player strives for their goal.
We don't know entirely how PP2 will work and if it will end up fixing these problems, but we know that no one is perfect and no piece of software was ever written that didn't have a bug in it somewhere in its first incarnation. These discussions are an attempt to provide enough context and visibility to them to make better decisions around things that related to potentially one sided or exploitable gameplay and, hopefully, prevent them from happening.
So no, by definition, utilizing one of the unbalanced mechanics to 'win' for your side in a manner the game framework / mechanics don't prevent may not meet everyone's definition of cheating, it may not be something FDev will publicly do anything about, however it is still unfair game play, and I don't see anything wrong with striving towards correcting that to make the game equally enjoyable and fair for everyone.