I thought it was more that we are just following the story in one of the very lucky universes that has no paradoxes.
Say there are LOTS (not infinite, but unimaginably large number) of universes. One for every configuration, every difference, every spontaneously created particle.
If a paradox is created, the universe ends. (or never was; depending on how you think about it).
We are following a story in one of the universes that did not end due to paradox.
In another one of these universes, harry continued with his experiment. This universe was never meant to be, and in fact it never was. Nobody was around in this universe to write a story about it.
In another one of these universes a toaster materialized out of nowhere next to harry. It stopped his time-travel experiment, and also confused him for the rest of his life. This story was confusing.
In another one of these universes our solar system was never formed. This story was dull.
In one of these universes something clicked in Harrys mind and made him impulsively send back a note saying "DON'T MESS WITH TIME TRAVEL" . This deterred further advances down this road, triggered a desire to send the note back and averted a paradox. This universe continued existing, and made for a good story.
There was an off-the-cuff line back in Ch25:
I wonder if there is something more to a magical twin connection, that may have even caused problems (confusing the source of magic?), or if this was just a comment on how dark/backwards things were in the old days.