It says why multiple times, right on the page parent linked to. They want Defold to be free to use, they are trying to guarantee it’s always available to developers free of charge, and that the source remains always available. Why do you ask?
I ask why because tons of other software achieve being free to use, free of charge, without preventing the software from being commercialised by a third-party. Why do they feel the need to prevent their software from being commercialized? The only reasonable answer seems to be jealousy.
The main repository (github.com/defold/defold) is the only one with the Defold License (ie the modified Apache license). There's also around 80 or so MIT licensed repositories with Defold engine plugins for things such as analytics, monetization, ads, attribution, game servers as well as tools such as Spine, Tiled, Rive etc.
They called Defold "open source" at the beginning and only changed the wording later when people started reacting, so this could be an artifact left from that.
