Is owning it enough?

This doesn’t affect me, but I’m still curious:

does having Source SDK Base 2009 (from l4d2 for instance) let us play BMS, or does it not include the same necessary binaries as 2007?

If you have L4D2, you should have Source SDK Base 2007 anyway, so it doesn’t really matter.

Founded in 2004, Leakfree.org became one of the first online communities dedicated to Valve’s Source engine development. It is more famously known for the formation of Black Mesa: Source under the 'Leakfree Modification Team' handle in September 2004.