On-topic on MSE includes things like (as quoted from the Tour):
- Understanding mathematical concepts and theorems
- Hints on mathematical problems (but please read our FAQ about homework questions)
- History and development of mathematics
- Solving mathematical puzzles
- Software that mathematicians use
In particular, the activity of delegating your mathematical work to others is off-topic.
The choice of what explanation to give was effectively put to a vote, and "missing context" won.
I do think it is unfortunately misleading, as adding details is merely a proxy for what is actually missing from postings such as the one you linked: e.g. a question about what specifically someone is trying to understand.
e.g. "What methods are there for proving a polynomial irreducible over a finite field?" is one possible question that a person might be seeking to understand by posting a question like the on you linked, and it would make a fine (and surely uncontroversial) question for MSE.
Another possible question is "How do you do arithmetic and algebra in finite fields?", although admittedly, a person with that problem who is asking a question like the one linked tends not to realize that's what they actually need to learn. (a point which is one of the pros for asking people to show their work)
(and, of course, "Will you do my work for me?" is a question that some people who post questions like that want answered in the affirmative)
Aside: while I strongly believe it is detrimental to MSE (both on its own merits and for the behaviors it encourages), there are those who believe that "exercise -> solutions manual entry" is a desirable thing, a point of view which would welcome questions such as the one you linked.