I think by "proven" he's implying that it's proven to himself, that is, that he's familiar with it.
I kinda like that philosophy, and didn't realize that I somewhat adhere to it: I tend to try to keep to tools I'm familiar with when I'm solving problems that I know are new to me. It lets me focus on the problem and avoid thinking too much about the tools. But if I'm in learning mode or need to implement something I understand well, I'll often reach for a new tool just to play around with it or try something different.
Yes, this is another aspect: I think there is only a certain amount of "new stuff" the brain can absorb in a given time period. You need to choose what to use that learning budget on.
I kinda like that philosophy, and didn't realize that I somewhat adhere to it: I tend to try to keep to tools I'm familiar with when I'm solving problems that I know are new to me. It lets me focus on the problem and avoid thinking too much about the tools. But if I'm in learning mode or need to implement something I understand well, I'll often reach for a new tool just to play around with it or try something different.