This paper isn't talking about a risk of an Earth impact, it's talking about a potential lunar impact:

> Studies of 2024 YR4’s potential lunar impact effects suggest lunar ejecta could increase micrometeoroid debris flux in low Earth orbit up to 1000 times above background levels over just a few days, possibly threatening astronauts and spacecraft.

Throughout most of human history, an impact event like this could probably have gone completely unnoticed, because we didn't yet have satellites that were vulnerable to micrometeoroid damage. So you can't use the fact that no such event was observed as evidence that it didn't happen.