Because adjectives are verbs in Korean (예쁘다 - to be pretty), there is a modifier used to make the descriptive verb into an adjective. That's what makes the 예쁜 form. 예쁜 개 - pretty dog. However, action verbs can also be turned onto adjectives. So in this sentences 먹는 is an adjective. You could add other words before 먹는 and make the 'adjective' longer. 김밥를 먼는 사람 -the kimbap eating person or the person who eats kimbap.
This is a difficult grammar point for me, so maybe this will help someone else who is struggling! If anyone has a simpler way of explaining it, please do ㅠㅠ
Here's the link to a lesson that helped me a lot when learning this rule:
I used to let the inconsistencies bother me, but keep reminding myself the course is in beta. It now accepts "The person who is eating." Over time, as more and more folks have made reports and offered alternative translations, the course has improved, though it is still a work in progress.
It is very common for Koreans to express a progressive or continuous action with simple present tense, but of course, there are scenarios where "the person who eats" would be the correct translation. I will keep pounding my drum for DL to add context clues to limit the number of possible translations.