This is something that is definitionally true in a finite universe, but doesn't carry a lot of useful predictive value in practice unless you can identify when the flattening will occur.
If you have a machine that converts mass into energy and then uses that energy to increase the rate at which it operates, you could rightfully say that it will level off well before consuming all of the mass in the universe. You just can't say that next week after it has consumed all of the mass of Earth.
