Earlier quoted context omitted.
The BSL is not an OSI-approved license, so it’s certainly not “open source” by the commonly used definition. I agree it’s a reasonable license. But it’s not an open source license.
The OSI is a consortium of cloud platform vendors (really - check for yourself). Of course they'll define open source in a way that excludes licenses that restrict them from turning your work into closed-source cloud platforms. The good news is that we're not beholden to their definition as they have no official status whatsoever. We don't have to believe them just because they put the words Open Source in their comp…
As to the specifics of SSPL, I personally don’t see the rationale for accepting AGPL but not SSPL.