I apologize. I unintentionally misspoke. You are correct that BSL is a Source Available license as opposed to an Open Source license.

I don't believe the website or the Github page makes that claim.

Many people's biggest issue with BSL is that there are dozens of versions of the "additional use grant" which each have bespoke language with very critical clauses, none of which have any case law behind them (correct me if I'm wrong).

Even though software may be licensed under "BSL" it isn't really a standard, even though proponents tend to use the term "BSL" as if it is the same as talking about "GPL" or "BSD". The MariaDB BSL used here is quite different than the HashiCorp BSL, for instance.

In this case the additional use grant is:

  Additional Use Grant: You may make use of the Licensed Work, provided that
  you may not use the Licensed Work for a Database Service. A ‘Database
  Service’ is a commercial offering that allows third parties (other than
  your employees and contractors) to access the functionality of the Licensed Work.

IANAL but to me that sounds like it could be interpreted as almost any commercial work based on it since some part will "access the functionality", indirectly or directly. I know that is not what they intended, but the language is loose enough to allow almost any interpretation.