1 Ron: But Hermione, we know from The Prisoner of Azkaban that time is fixed in this universe.
2 Ron: You can't change what happened in your past.
3 Hermione 2: Oh Ron, don't be so logical. You know, you really annoy me sometimes. Unlike...
4 Will: {arriving} Hermione!
4 Hermione: Will! {love heart symbols in text}
4 Hermione 2: Will! {love heart symbols in text}

Fixed time is one of the possible ways of theorising how time travel might actually work. In a fixed-time universe, you may be able to travel back in time, but you can never change anything that actually happened. Whatever you do in the past turns out to have been what you already did, no matter what you may do to try to prevent that.

This is actually quite a popular form of time travel in fiction, since it leads to the creation of stable time loops, which is one of the major ways of avoiding having to deal with the paradoxes that arise when you allow history to be altered.

There are of course other theories of time travel, in which the past is not fixed, but I don't want to go into those today.

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2020-02-22 Rerun commentary: We also know from this strip that Ron has meta-knowledge of the stories that they are in, since he cites The Prisoner of Azkaban by name.

I just realised that in that book Ron is completely oblivious to the use of the time-turner, being injured and resting in the Hogwarts hospital wing while Harry and Hermione travelled back in time to ... do those spoilery things. Although one presumes that Harry and Hermione would have filled him in later, so I suppose it's okay that he knows about it here.