Based on the information you give, the add-in is absolutely free and open source. The "free" part is the fact it is licensed under the Apache license. And it is open source as they are offering the source code.
The fact that the add-is made for a non-free closed source application is irrelevant. That application is a completely different entity and has no bearing on the add-ins "freeness" or it's source. The same goes for the programming language. You are able to take the add-in and redistribute it under the terms of the Apache license and modify the source code which is given.