you have to look at the context. as someone answered, it is usually used that 저녁'을' 먹었다(=i had dinner) or 저녁에 보자(=see you at evening.) but it can be also used 저녁에 먹었다 (=i ate something at dinner)

No, because in this context it means the time at the day. You can see it because the time/location marker 에 is used. If one are talking about the meal. one wouldn't use 에 and might also add 을 instead. So in this context, something that may or may not be dinner, is made in the evening (it doesn't mention what that is made).