As someone who has suffered a bout of bilateral tendinitis and a bulged neck disk, let me blunt and say: bullshit.
Now, I'm not telling you that there are not forms of RSI that are some, weird, self-defeating psychological thing. And certainly a positive attitude may help with recovery. But do not run around telling people with real, physical issues that it's all in their head.
Unfortunately the opposite is a much, much bigger problem right now: people running around telling people with chronic pain that they must have physical issues that must be corrected, and that pain that is not caused by physical issues is somehow not "real".
There is a lot of science about this. Look up David Butler and Lorimer Moseley if you're interested.
Now, I'm not telling you that there are not forms of RSI that are some, weird, self-defeating psychological thing. And certainly a positive attitude may help with recovery. But do not run around telling people with real, physical issues that it's all in their head.