The two are not mutually exclusive. We all know endless growth with finite resources is impossible. We can innovate do things more efficiently or differently

The two are not mutually exclusive. We all know endless growth with finite resources is impossible. We can innovate do things more efficiently or differently and find solutions to previous limits of course. The hypothesis in the paper is simply that eventually a civilisation will encounter circumstances in which the timescale to solve it is longer than the time they have to do it. Then they must either achieve a form of homeostasis (and no growth) or die off (or retreat and start again). The boundary for this doesn't have to be a planet. It could be a solar system, or maybe more than one.