"They tell the story."
Translation:Insíonn siad an scéal.
In standard Irish, synthetic forms are only used for the first person, for the most part (in the present tense, at least), so you get insím and insímid in the first person, but insíonn sé and insíonn siad in the third person.
Munster Irish used synthetics forms much more widely, and the synthetic form of insíonn siad was insíd. (You will still encounter it in Munster Irish, but even in Munster, the analytic forms are now often used).
At some point insíd was added as an acceptable alternative answer for this exercise. Due to the vagaries of how Duolingo was designed, when you submit a wrong answer, Duolingo selects the acceptable alternative answer that is alphabetically closest to the wrong answer that you submitted, even when, as in this case, it is a bit misleading. Duolingo should have accepted insíd if that's what you submitted, but it should not have suggested that you use insíd.
There are similar synthetic forms for all or most verbs in Munster Irish, it's not just a feature of inis.
insíd is a synthetic form of insíonn siad that is mainly a feature of the Munster dialect. It's not "the" right answer, it's "a" right answer, and Duolingo probably shouldn't be showing it to you, as it causes more confusion at this level than it's worth, but when you submit a wrong answer, Duolingo does a crude alphabetic search to find the "acceptable alternative answer" that is closest to the wrong answer that you submitted.