With auxiliary AVOIR:
The past participle is invariable EXCEPT when the direct object is placed before the verb. To identify the direct object, ask the question: VERB - whom/what ?
Example: I love my son - I love whom? - my son -> my son is the direct object of verb love.
j'ai écrit (past participle) une lettre (direct object) -> invariable
la lettre (direct object - feminine singular ) que j'ai écritE (past participle) -> agreement to feminine singular direct object.
est-ce Marie (direct object) que j'ai vuE ?
les œufs ? je les (pronoun replacing direct object) ai mangéS.
celles que j'ai mangéES (celles is feminine plural)
If you had said "Are those the ones that I ate?" you would have turned what is strictly only a partial sentence into a full, complete, grammatical sentence in English, which means what the French is implying. (That's what all our teachers used to insist on a generation or two ago!) But Duo is very conversational, so we have to try to write as they speak, I think.
The words "these" and "those" are often used as limiting adjectives (not just any apples, for example, but "these apples"). However, when "those" or "these" are used without a specific noun following them, the noun is implied. For example: Q: "Which ones do you like?" A: "I like those!" I'm not sure if saying "those ones" is technically incorrect, but I doubt you would ever hear it said by a native speaker.