The reason is that "il lance" and "ils lancent" are homophones. The recognition system is based on written sentences, so plurals may not be recognized as correct in dictation. A new system will soon allow for all homophones to be identified by the machine. In the meantime, I have disabled the audio for this sentence, so that learners don't get it in dictation again.
Try a dictionary. You can add an app to your mobile device if you like. Both "lancer" and "jeter" can mean "throw" but there are some slight differences.
- jeter : 1) throw/toss, 2) throw out/away, or in a sense that involves a movement of the body, e.g., jeter un coup d'œil (or) simply jeter un œil à quelqu'un (to glance at someone), 3) to jot/scribble (a note), and more. http://www.larousse.fr/dictionnaires/francais-anglais/jeter/44818
- lancer : depending on context, it can mean throw/toss (e.g., a ball), fire or shoot (using an instrument of some kind), drop (a bomb), to make (a joke, e.g., lancer un bon mot), to stab, and more. http://www.larousse.fr/dictionnaires/francais-anglais/lancer/46054
"une lance" (noun) is a spear, not the verb "lancer" (conjugation: like "manger")
In the absence of a determiner (article or possessive or demonstrative adjective) "lance" with a personal pronoun just before could not be a noun anyway.
Maybe you should not rely so much on the translations given, they can't substitute a good dictionnary. If you are working on Duo with your computer, you may open another window and find a free online dictionnary to check on words as you go.
From listening to Montreal Canadiens hockey games in French, they used to always say, "Il lance, un but!" Basically meaning "he shoots he scores" You shoot a hockey puck with a stick, but you are really launching the puck toward the net.
In baseball, the pitcher is called "le lanceur" (one who throws). Also in baseball, you can "throw out" the first pitch of the game (A ceremonial act to commence the game) I'm not sure if you would use lancer. And of course, you can "throw out" the trash, but that would probably be some other verb similar to "dispose of" rather than "lancer." I'm not sure about that, though.
I was wondering that, too. I typed "he launched the hat" and it told me it should have been "he launches the hat." But when I typed "he launches the hat" next time, it said I was wrong and should have written "he tosses the hat." When i hover the mouse over the word "lance," though, it specifically says "launches/is launching."