Learn Russian: Что они писали в длинных комментариях? - What did they write in the long comments?

The dictionary form is комментарий, which is a masculine noun ending in -й. In the prepositional plural, nouns ending in -й or a soft sign take the soft ending -ях. Words ending in a regular hard consonant take the hard ending -ах.

Why does the adjective длинных end in -ых? Could it be -их?

Adjectives in the prepositional plural take either -ых or -их. The dictionary form is длинный. Because the stem ends in the hard consonant н, it takes the hard ending -ых. You only use -их for soft stems (like синий) or when spelling rules require it after certain letters like г, к, or х (like русских).

Doesn't the preposition в sometimes use the accusative case? Why is it prepositional here?

Yes, в takes the accusative case when describing movement or direction (going into something). But when describing a location (being inside something), it takes the prepositional case. Here, the text is located inside the comments, so we use the prepositional plural в длинных комментариях.

Why use the imperfective verb писали instead of the perfective ?