It is still a group that make close source software, they limited it to just some developers, is that not centralized when the public can not see the source code used in the wallet makeup in a way the public can check for vulnerabilities.
Again, you are talking about a process. It can be said that the process of software development is decentralized when the code is open-source, and it is centralized when the code is closed-source. In the case of open-source, not only is it possible to verify everything for yourself, contribute to it or something like that, but also you can fork yourself away from the original implementation and start building your own version of a wallet. Essentially, open-source software allows for decentralized free markets for development. On the other hand, when software is closed-source, individuals can't follow and influence the course of development, nor can they run their own version of the same implementation, which makes them dependent on the decision of a small group of people. It is not a free market, but rather central planning, which bitcoin supporters should be squarely and totally against.
The bitcoin network is decentralized because the public can be able to participate in the network, anyone can be a miner, anyone can run a node and like that, but a wallet organization that deprive the public from view the source code of the wallet for fixing vulnerabilities and the likes, just having their own developers to view the code and check for vulnerabilities should be considered as centralized as wellet.
If a time will come a governmental body will secretly dialogue with close source wallet developer to include some malicious codes that can spy what the wallet users are using which something like that can happen. I think close source wallet should perfectly be considered as not decentralized.
You're absolutely right: even if it was possible to build a decentralized network on top of a closed-source foundation, those networks would be very fragile and wouldn't last. The variety of implementations wouldn't provide the same resilience as we have today in bitcoin, it would only postpone its inevitable demise. But anyway, in purely decentralized systems you can't prevent people from building closed-source software nor can you prevent them from running it.