It is very misleading claim that bitcoin users do not need to trust centralized authority, in fact every one uses bitcoin is trusting this centralized protocol originally designed by Satoshi: Every miners, nodes, exchanges, merchants, users, no exception
One does not have to "trust" a protocol. Actually, in the case of bitcoin, there is no protocol separate from some version of bitcoin software. One can inspect that software and see in detail what it does and how it does it. One can build the software from source code if one doesn't trust the downloadable binaries. If one lacks the necessary skills to understand the source code one can hire people to do review on your behalf.
Now contrast this situation with you you face as the user of the fiat banking system, or a fractional reserve bitcoin exchange such as Mt. Gox.
What you said is very true, bitcoin's open source nature is its biggest advantage
However, when the protocol become 1 million + lines, I guess your trust is not so easy to establish if you try to inspect the code by yourself, you start to rely on other more involved devs
This is especially the case for most of the users without IT expertise, they just blindly download a wallet recommended by bitcoin.org and never question the validity of those clients, and they blindly upgrade to the latest version without reading the change log
Full nodes are more careful in selecting software, but still I doubt that those node operator really go to Git and read the source when they apply an upgrade. Typically they just read change log, and even if they do not fully understand a change's details, they still applies it based on the "Claimed" higher performance, stronger security, or newer feature etc... e.g. they put all their trust on core devs, believing that they will not screw it (And that's the reason we had 2013 hard fork incident)
Unfortunately currently the split in the core devs started to change all that trust. But then the question is: If you can not trust core devs, who can you trust?