That is only true if assumed that the rocket will not gather additional energy on the trip.
Also the premise that the energy has to be concentrated to a certain are for the rocket to work is wrong. You can have as much energy on as large area as you want without reaching the critical mass and collapsing into a black hole. The rocket can be of any size to work.
Even so, a black hole is the perfect source of energy. Converting mass into energy without loss via hawking radiation. And the mass is collected when moving trough interstellar medium. Such micro black hole( it has to be small to radiate fast enough ) would still weight a lot and moving it would be difficult, but not impossible.
Also the premise that the energy has to be concentrated to a certain are for the rocket to work is wrong. You can have as much energy on as large area as you want without reaching the critical mass and collapsing into a black hole. The rocket can be of any size to work.
Even so, a black hole is the perfect source of energy. Converting mass into energy without loss via hawking radiation. And the mass is collected when moving trough interstellar medium. Such micro black hole( it has to be small to radiate fast enough ) would still weight a lot and moving it would be difficult, but not impossible.