Yes, I know the main page mentions that it's an unrestricted free license, but that's not actually saying what the license is. There's a price tag on the engine, so it's clearly not copyleft. But the only thing I can find anywhere in the files is the license information for the dependencies. There's even a message at the top of that file discouraging people from actually reading it.

How can you call this a "professional grade" engine and not tell people what they're allowed to do with it? Admittedly, I would only be interested in a making a game with it and perhaps selling the game, but that's only because I'm not sure the engine is easy enough to use. Just last week I came to the understanding that it was fine, according to the license, to hand a copy of Pico-8 to a relative that lives with me so she could dabble in it a bit. I can't tell, even after scouring all the files, whether that'd be legal for me to do with this engine.

We’re indies just like y’all. You’ve purchased the engine and can use it for commercial purposes with 0% royalties. The standard version is a one time purchase and the Pro version is a yearly subscription. Regardless of which you choose, you have to write to use the version of the engine you’ve purchased (the subscription is for receiving updates/newer versions).

We’ll put that in a EULA.txt file.

But the bottom line: we’re not trying to screw you over and want you to succeed as an indie.

You should only use the engine if you believe the statement above.