"The food is cold."
Using the word "al-akl" sounds rather unnatural. I think the word "ta-am" would be more natural sounding.
I agree, they are sometime interchangeable, but "al-akl" is usually used to refer to the act of eating rather than the food itself; which is الطعام "ta-am" like you said.
Examples : Eating is hard = الأكل صعب (al-akl sa-ab)
We use it quite naturally this way in this context in Lebanon. The course makers aren't trying to teach pure MSA here. They're trying to strike a balance between MSA and spoken language. As they mentioned in an article linked during the course launch:
we’re teaching a less-formal, spoken version of Modern Standard Arabic — not the version that would appear in poetry or formal news broadcasts, but instead the version that would be used once a newscaster stopped reading from their script and started talking to their interviewee. It’s a version of the language that can be used in a formal conversation, but one that also can be used with the widest range of Arabic speakers.
You can read their full article here https://making.duolingo.com/what-makes-arabic-hard-and-why-that-shouldnt-stop-you-from-learning-it