This is one of those yeah-but-actually-no things. An accent is either
- A set of pronunciation, stress, and intonation features which reveal your native language, or native language variety, usually spoken in the area of a single country, when you speak in some other language.
- A set of pronunciation, stress, and intonation features which reveal your native language standard version, usually spoken in an area of a single country, in comparison to other such areas.
Accent is NOT the same thing as dialect. If you have two people from the UK, one from Liverpool and another from a small village in Yorkshire, conversing in English, both of them have a British accent, although they speak two very different dialects. If you have two people from the States, one from Iowa and another from Texas, both of them conversing in Finnish, Spanish, or any other language that is not English, both of them have an American accent, although their dialects are very different. In fact, it would be almost impossible to guess from which part of the States they are simply based on what their Finnish sounds like. :)
I came to the forum section because I thought the same as you, btopps. "They have an American accent" is a little awkward in native English (I grew up in England). It's perfectly correct, but just sounds off.
In the real world, away from Duolingo, if someone asked me "Where are they from?" whilst gesturing over to a group of people, I'd natively respond with something like "They sound American", "They've got American accents", or "They might be American." "They have an American accent" wouldn't really ever bubble up to the surface as a potential answer for me.