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Doesn't mention anything about open source.

Did you even bother looking? It states on the linked page that the code is under the Apache 2 License and the download page states it too:

Community Edition FREE

Lightweight IDE for Python development only

Free, open-source, Apache 2 license

Intelligent Editor, Debugger, Refactorings, Inspections, VCS integration

Project Navigation, Testing support, Customizable UI, Vim key bindings

I suppose Meai is referring to AppCode, which is a different product.

If so, my bad. :-)