Not the author, but this prevents leaking all URLs you are visiting to agolia. Instead of asking Was this URL submitted to Hacker News? you ask Which URLs have been submitted to Hecker News?, store that in a Bloom filter, and then use the Bloom filter to answer the question Was this URL [likely] submitted to Hacker News?

Is the story being submitted a common feature like dupe detection or maybe unsubmitted link detection?

How does the filter know anyways?

If it is provided by the package, one could imagine adversarial scenarios where "high interest" submissions are excluded from the filter(maybe even make a custom one each time that somehow lands them in a collision) so Algolia knows exactly what it wants to know about you.

Probably, I should have read more into both of the projects before asking.

I realized a collision wouldn't help because the client would just avoid sending a request as in the normal case.

The malicious filter would have to conspicuously avoid tracking the URLs of interest.