Thanks for bringing this question up again. I think it is reasonable to discuss it again to determine how the community thinks of this now and how we agree to handle it. Maybe it's time to reconsider how we apply our own standard and make the helpcenter reflect the practise.
This ist the most recent discussion on the topic (early 2016): What defines the borders between RPi and other related Stack Exchange sites?
Personally I used to go with:
When in doubt, ask there first. Questions where the Raspberry Pi is not a significant factor will likely be closed. [emphasis mine]
However as a moderator I try to not hammer things down without a few votes by the community (sometimes I still do if I feel it is the right thing to do or if the question is blatantly off-topic). So usually votes need to be carried by community - that is, five voting members to agree an a certain position. So as Goldilocks' has put it in his answer to the meta question linked above: "this depends of the willingness and availability of participants" and might take some time.
While we need questions to be at least a little specific to the RPi Milliways has brought up an important point (again in the discussion linked above):
I agree, although pragmatism sometimes prevails. If a new user asks a 'NIX question I will often answer, because the questioner is often new to LInux and the Pi is the first and only experience. Besides Unix & Linux (Operating systems) can be intimidating. Users there don't seem to understand absolute beginners and confuse them with technicalities.
This is usually fine if a question receives a helpful answer at our site. If it does not however, one might still consider asking at the larger and more general sites such as SO or U&L. Note that migration of such questions is not always the best approach. Usually hinting to ask alsewhere while closing a question here works better if the question need some polishing to work well on another branch of StackExchange.
I have experienced that most users of our community tend to give quite some leeway to questions of users that are new to the Raspberry Pi and/or StackExchange... and again if the question does not blatantly violate the scope of our site and the respective user gets a helpful answer here we could call it mission accomplished. Given time and a few guiding comments one could still point out the nuances of the workings of StackExchange and how and where to post additional questions.
The openCV question that you have linked however is an example of how it's not done well. It is technically completely unrelated to the Pi and a solution that would perform well on any other Linux board is likely to do the trick on the Pi too. The question received just one short answer and one could very well argue that the significantly larger user base on SO could have been beneficial to better answers (or impromptu close as a duplicate).
Closing it now however will change little as it will still persist on our site (question with answer). It is also too old to migrate away. The only good that it would do is a sort of setting a precedence here... but I think this is done better discussing the issue here on meta and handling new questions.