It's opensourced under Apache 2.0 license. That means, it's free as in freedom, with all the legal ability to be forked and profited from. AFAIK, the only substantial difference between Apache 2.0 and GPLv3 is that APL2 is not copyleft license. That means, one's fork is not even required to remain opensourced (as far as it contains a reference to original APL-licensed version).

So either Mortar opensourced some feature-crippled fragment of their platform, and it relies on features from their proprietary platform heavily; or the statement of requirement Mortar account is property of the Tutorial's approach, not the opensourced code itself.

It's called the GPL. If that's really the model they're trying to create, it would be nice if they just used the GPL.