Is there any proof that they can do that?
I guess, they are just full of bullshit. Who are these guys? Are they know in the Bitcoin World? What Proprietary Data is there in Bitcoin? It's open source and an open ledger.
You just have to look into the Deep Web and find thousands of people, who claim such things
Why would they not be able to do it? I mean you can monitor all altcoins if you want to, however software is required if you want to actually do something with that information.
I'm not sure exactly how effective it might be. However, the easiest way to get information would be once a suspicious address sends coins to a exchange such as Bitstamp and request information from there.
Now, my question is where are the Monero promoters. They should not be able to track people using it, so it can be used to clean your money/hide your tracks.
I did not state that. There are ways of attaining information, such as subpoenas.