Wordreference seems to argue that aimer something can mean like or love but aimer a person is love unless you say aimer beaucoup (or bien).
Linguee says you can translate it as loving or liking someone or something.
Cambridge dictionary follows Duolingos construction.
So all I'm saying is in real life someone will use aimer and depending on the situation it might be like or love, and it may not only depend on if they're talking about an object or a person. Duolingo has set up they're rules to be consistent and accept the most common usage and that's probably good for learners. I still think it's important to remember that language is not math and even if aimer has a very clear and distinct definition in French, "like" and "love" may still be flexible enough in English to meet that definition. Language is almost never as simple as "that's how it works".
For the most part I think Duolingo does really well helping to understand the complicated nature of language. Use words how you want, but now you know how Duolingo is going to grade you :).
I've got a longer post in this thread explaining in more detail, but cela is going to refer to an object normally and not a person. Doulingo holds the rule that "aimer" something is to like it and "aimer" someone is to love them. Like I said in that other post, I feel like there's more wiggle room in the real world but at least Duolingo is consistent.