Yep, it is written plainly in the article.

Originally Posted By: Scienceblog

It appears that the Legionella bacteria (Legionella pneumophila) can thrive in the warmed water that is held for the windshield washer system


snip...

Originally Posted By: Scienceblog

The chemicals that are often added to wiper fluid to improve cleaning appear to be sufficient to prevent growth or survival of the bacteria.


Technically I could use only plain water in the summer - but why on earth would I do that? Wiper fluid is all-season stuff. All season stuff means one thing less to worry about...

And let's not ignore the fact that wiper fluid does a much better job.