Customs-related questions are normally considered on-topic here as long as they relate to travel (customs & border crossing are listed as topics we cover). So questions about bringing items with you as a traveler, and the regulations or duties that apply, are on-topic. These are often tagged import-taxes or the generic customs-and-immigration. We've had a number of questions about traveling with PCs that have been on-topic, such as the similar Do I need to pay import duty for bringing disassembled desktop computer to USA?
However, questions that are strictly about import/export without a reasonable connection to short-term travel are indeed considered off-topic. Examples would be mailing goods or managing commercial shipments of merchandise. We do have some shipping questions (shipping) where there is a travel connection, such as unaccompanied luggage transport or mailing shopping home from a trip, but many shipping questions are closed, as they're either an Expatriates matter or just not about travel.
In the case of the question you cite, I think there's a fairly good argument that it's a travel question. The asker is traveling themselves, they don't seem to be traveling solely to transport their computer, the goods are more of a personal nature (1 PC, not a whole rack or something blatantly commercial), and the items aren't for sale. In other words, the OP is bringing this with them while traveling, and wants to know what rules apply. To me, that's on-topic here. One place where it gets more interesting is whether this is short or long-term travel. That could be relevant to giving an accurate answer (different customs rules usually apply), and also to whether the question belongs here or Expatriates. I'd ask in comments to clarify that.
If you do see an off-topic import/export question, I'd vote to close as "Off-topic—Other," and then write a comment explaining it's off topic because it's not about travel.