I think when people use this expression they're generally talking about motivations/beliefs/interpretions of events. It's only truly mentally ill people that are using it in a literal sense (eg. their 'truth' is that they've met the Pope when they haven't).

That expression irritates me too though. It's obviously valid to say event A leads to action A in one person and action B in another, due to different and subjective interpretations of previous events. But my experience is that on the most part, people who come up with that line are using it as a fluffy rationalisation for things they know don't make sense and/or they shouldn't be doing, and aren't particularly interested in leaving that comfort bubble.

And string theory does almost, sorta, maybe, show that - but two big things to bear in mind:

a. Those externals things do then exist externally, which is very different from an hallucination of some kind.
b. String theory though interesting is poorly understood and so far mostly unproven.