Many MO questions, not just reference requests, could be answered by a literature search. Yet, a main point of having this site is to avoid having somebody spend a lot of time searching for something that somebody else just might know from the top of their head.
However, it should also be note that the emphasis here is on "a lot." Before asking a question on this site, any question not just reference requests, one better should make some effort oneself to find the answer by oneself, and this is also in the FAQs, more specifically in How to write a good MathOverflow question?How to write a good MathOverflow question? (under "Do you homework")
Thus, this site should be used to avoid long literarture searches, yet not to avoid all literature searches.
It is difficult for me to tell, this not being my field, how hard it is to find the answer to that particular request linked in OP. For me, as a non-expert, the request seems well-written, especially I like that it mentions the related result for Hilbert spaces. And, I would thus assume, or at least give the benefit of the doubt, that OP made some effort beforehand and thus find this particular question a rather good example for a reference request question.