It is the minimum thought to be possible for a black hole produced by standard stellar evolution.

GR has no limit, large or small, on the size of a black hole. Such black holes could be formed by another mechanism -- in the big bang ("primordial" black holes) or from the merger of neutron stars.

What has everyone atwitter about this is that, given the standard plan for things, these objects should be very rare. To have seen even one example this early in the history of gravitational-wave observation is a surprise.

Either there are more primordial black holes than people think, supernovae don't work like we expect (historically a pretty good bet), neutron stars find each other and merge more often than people think, or there's another process for making mass-gap objects that people don't know about.