I think it's simply a way to distinguish between old style JS ecosystems and new. It encompasses a range of benefits and lessons learned in JS. Of course not everyone is going to agree that the "modern" way is the right reaction to the problems of the past but most active JS programmer will have a good idea what it means.

C++ has a similar issue. There's a very distinct difference between C++11 and what came before. And C++20 could be another big shift (especially with modules). Modern has a useful meaning, even if it's fuzzy and temporal.