after some reflection, I find the question not suited for the site.
Is it only me?
No, it's not only you. I also think that question, and questions like it, are not appropriate on the site.
However, I find the guidance on this site's help pages less-than-clear on the topic (sorry, pun not intended). From https://aviation.stackexchange.com/help/dont-ask, we find:
You should only ask practical, answerable questions based on actual problems that you face
I find it difficult to believe that the question was asked out of anything other than idle curiosity. I doubt that there is an actual problem the author of the question faced that led to the question.
Similarly, nothing on the https://aviation.stackexchange.com/help/on-topic page listed under examples of on-topic questions suggests to me that questions of the form "why was this airplane at this place at this time?" are on-topic.
On the other hand, examples given of off-topic questions focus primarily on the misuse of the site as a discussion forum, and/or for questions that are purely opinion-based, or more properly asked and answered on a different, more specific SE site. So nothing in those pages also clearly says this type of question is off-topic.
Additionally, there seems to be fair precedent for "plane-spotting" type questions here on the site. I don't personally think that they fit the "on-topic" mold, but given that they are tolerated, I think there's not much clear justification for prohibiting a particular "plane-spotting" question on the basis that it's unlikely to be answered. After all, where do you draw the line? How do you even determine where the question is relative to that line?
If the site is going to allow plane-spotting questions at all, I think it has to allow all plane-spotting questions, regardless of the likelihood of an answer. Because no matter how remote the chance of an authoritative answer might be, the possibility still exists. And I don't believe there's a good, objective way to decide the likelihood of an answer, nor what the right threshold of likelihood is for allowing such questions.
That said, I do think that any answer to such questions must be factual. The currently posted answer to that question is not, and should be down-voted, if not deleted altogether. It's purely speculative and provides no real insight as to the purpose or reason of the actual flight in question. It doesn't really answer the question, yet its presence may discourage others from making an attempt to answer the question in a factual way (e.g. do some research, such as that which the author of the question should have done themselves before posting, to learn the nature of the flight).
If we are going to believe or at least accept that such questions can be on-topic, then we should also at least be rigorous to reject answers to such questions that don't really answer the question.