Could you answer me about this excerpt I found on a book I read?

“We are supposed to believe that the depleted remains of the star collapses into an extremely dense pellet that has such a high gravitational pull that light cannot escape from it and that its gravitational pull can suck in much heavier active stars and eventually form much bigger and more dangerous black holes. I do not understand how they can justify that a burnt-out star that has suffered a substantial loss of mass and size can create a super gravitational force with amazing space-time properties. I have never heard of gravity being affected by density instead of by mass.”