More generous reading of the entire sentence would be that the usable life of the mmrtg is increased by improving the energy efficiency of the rover. The mmrtg power output is constantly decreasing, and so it is reasonable to say that the mmrtg reaches end of life when the power output is not enough to operate the rover. So that cutoff point depends on the power demands of the rover.
Yeah. There's nothing wrong with that statement. Reducing the minimum power required will obviously prolong the life of a system that has a monotonically decaying power supply.
The problem with being nitpicky is that fixating on isolated/arbitrary details often just means missing the bigger picture in a way that's even more incorrect. Good for "gotchas", but not intellectually productive.