Mankind didn't know about germs, black holes, electrons, radioactivity, … for a long time but back then no one ever claimed they didn't exist, either. They were simply outside our realm of experience but at the same time there was also nothing in our experience as humans that was really constraining their existence.
Today, we have a pretty good understanding of the broad strokes of how nature works (think local energy & momentum, spacetime, quantum mechanics, etc.) Yes, we don't understand everything yet (not by a long shot) but we expect future insights do be compatible with what we've seen and measured so far[0]. In that sense, today's "laws" of nature constrain tomorrow's (updated) "laws" of nature. This is also how the authors arrives at his conclusions.
[0]: Yes, there is no guarantee for that. It could happen that physical laws change wildly from one day to the next. But this is not a particularly interesting direction for a research program, because if you start with that assumption, then anything could happen at any time, and physics as the discipline of trying to understand nature would become a rather pointless endeavor. However, the success of physics at describing nature seems to indicate that we're onto something.