I understand your struggle, but I'm afraid there is not really one answer to rule them all. That's why I always refer to that article.
For me personally, if I have to give a definition:
Customization is changing OOTB records which are captured in update sets.
Configuration is everything else.
So in your example it would mean that if it's an OOTB flow, its customization, and if it's a custom flow, it's configuration.
But this is probably one of the thousands of views you can and will get around it. So although it might be hard.... Forget about it 😉
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