I think you're right. Their summary says:

> You are free to modify Defold and you are not required to share the changes

but also:

> You can not commercialise original or modified (derivative) versions of the Defold editor and/or engine

I think they mean you cannot start a business selling your fork of their game-engine. I don't think they mean you may not use a modified engine in a commercial product. This seems to fit with the full text of the licence, which says:

> You do not sell or otherwise commercialise the Work or Derivative Works as a Game Engine Product

I think it would be pretty hard to distinguish between commercializing a game with a modified Defold engine which can be heavily modded by the player and commercializing a modified Defold game engine. Quite a few games have been modded into entirely different games sharing only the engine.

Just something to think about if you were planning on allowing mods in your game. Do they only care about how you advertise the product, or will they be looking at how it's actually being used?

Interesting point. That's different from typical moddable commercial game/game engine.

Typically, people may buy a game in order to play its free mods, but you can't tweak the engine and then sell that modified engine to other game studios. Whatever happens, the end-user has to have a licence for the original game.

This is a little different though, as you can modify the engine, then sell your game, and permit payware mods to use your modified version of the engine, perhaps even insisting on taking a cut. All the while the Defold guys don't get a penny.