Translation:Flirting is most effective in Esperanto, is it not?
Note that the primary meaning of flirti is more like "flit" or "flutter" -- flagoj flirtas en la vento and a butterfly will flirti from flower to flower. It can be used in Esperanto to mean "flirt", but it carries the nuance of casual flirting without serious intent -- what I would call "playing the field."
The more neutral word, well established and more common in Esperanto "in the wild" is amindumi.
I have brought this to the attention of the course authors, and in spite of comments from other experienced Esperanto speakers confirming this, it seems the course authors have decided to stick with "flirtado" here.
I was already married when I started learning Esperanto (my wife and I learned together) so I have no first-hand experience in this area, but given the number of couples I have met over the years and witnessed becoming couples, I will assert that at the very least there is a rather large kernel of truth in this sentence.
If you're young and thinking you might like to fall in love some day, Esperanto is not without its perks. It's not called Edz-peranto for nothing.