Yes, but "elderly" can be a noun to mean old people (note the plurality here). I cannot find a definition - even on line - to refer to a single old person as "an elderly".
I suspect that Duo might have made a mistake here, but English does change rather rapidly and so it might just be a new usage.
Interestingly, I have heard "old" used as a noun to mean an old person, singular, but that was always derogatory, and I cannot find a definition allowing it to be a singular noun.
Why is 'elderly' accepted when 'elderly' is an adjective, just like 'old'
See several of these quotes: https://www.brainyquote.com/topics/elderly Yes “elderly” is being used as shorthand for “elderly people” but it is being used as a noun. I have never seen it used as a singular noun, but then up until this translation I had never seen (or at least noticed) the adjective “senior” being used as a noun.
A question on pronunciation: the recording sounds to me as though the speaker divides the last word as follows: mal-ju-nu-lo. However, the word is built: mal-jun-ul-o. Is doing this common? Is it no big deal? Are there cases in Esperanto similar to this English sentence where scrupulous pronunciation is absolutely required: "They had vast plans, but only half-vast execution."