It's radio, the broadcasting system, not radio players, radios, etc. I look at this and I'm pretty sure what it really means is that he's slow on the uptake, somewhat retarded. It's not meant to be taken literally. That's a very unflattering word for slow in the sense of late, dull, etc.
Duolingo needs to reflect on its purpose a little more. After my first 6 months in Korea, I didn't care about slow radios or Internet memes. I struggled to ask for the check at a restaurant or make a deposit at the bank. Maybe we could get something like "Their tellers are especially slow?" ....
The first time I got this question I was translating "His radio show is especially slow" from English to Korean and the accepted answer was 그의 라디오가 특히 느려요. Now translating it from Korean to English it won't accept "radio show" (DuoLingo's original answer). I'm assuming this is the type of thing I should report?
According to krdict.korean.go.kr/eng the first meaning of 라디오 (noun) is:
In a broadcasting station, the act of sending sounds through a radio wave to enable a person with a receiving device to listen to them; or such a broadcast.
This is quite different from how the English word radio is used, which is either as a noun (the receiving device with a built in speaker) or as an adjective, i.e. radio waves, radio frequency, radio broadcast, etc. The Korean word is always a noun, and it can mean "radio broadcast" or the act of sending sounds via radio waves. So the English translation "radio show" should have been accepted.