than was useful = than (the knowlege) was useful?

I am assuming that this article is about some kind of search engine in the process of being designed.

To lower the threshold means to set up the search engine to search threw a larger set of sources then perhaps necessary. Doing so would then flood the user with too much information, much of it irrelevant and useless.

You could rewrite this sentence as such:
You could lower the threshold, but at a certain point, the resulting flood of information would be counterproductive.

It's not the knowledge that's not useful but the pile of useless knowledge on top of the useful knowledge that makes the problem.

The construction "more [...] than was" indicates that some kind of level or quantity was exceeded, in this case the amount of knowledge that was useful. If you lowered the threshold then you would have more knowledge than you could make use of.

Consider the following:

My recipe needed 5 kilos of flour. You gave me 10 kilos. I had more flour than was required.

The train ticket to London cost 20 pounds. I had 15 pounds in my pocket. I had less money than was needed.

@tsarstepan, he's not asking about the meaning of "threshold" but about the "more ... than was useful" construction, I think. Maybe he will clarify.

Both are informative.

Thank you.