Yes indeed, that is correct. This means in practice, that if you are going on a long trip, then on a UK car you would activate the REx engine at 75% battery and set it to "maintain charge".
The engine will do its best, but its 34Hp means it will constantly drain battery if you are, for example, travelling at speed. I understand it will manage to maintain battery only up to about 50mph.
Perfectly fine for extended city driving, but not at all good enough for a long motorway (freeway) journey IMHO. It really is not a car for very long journeys, and nor was it designed to be.
It is a great car when used for its designed purpose
I am surprised though that BMW do not allow the engine to charge the batteries above their level (rather than maintain charge), where power permits. And to charge the batteries when the car is parked. That would have been useful when stopping for a break on a long journey.
It would be inefficient of course, but useful.
These are just some of the reasons I went for the BEV version.