How's it different from AGPL? It seems like AGPL creates certain restrictions on the internal order of a company which uses AGPL code for the benefit of the AGPLed code's community. And this licence is basically the same.
If MongoDB were AGPL, both a company providing MongoDB-as-a-Service and a non-profit web app using MongoDB in the backend would equally need to publish their source code.
The BSL just says "you can't do this one specific thing for 36 months".
The AGPL is a more viral version of the GPL, which basically says "if your code uses this code, your code is now GPL licensed" and also "for all GPL licensed software you distribute, you must also make its source code available" (with the AGPL just redefining that "distribute" as "distribute or make accessible as a service").