I'm not sure if your flag should have been considered unsupported by evidence, but I think even this decline reason was reasonable.
(As random says, it had to be declined even if there was good evidence to support it, so I'm talking about something a little bit different from the question of whether or not it should have been declined. Furthermore, if that decline reason was borderline applicable but not really the reason the flag was declined, a custom reason would still have been better.)
On the one hand, as daramarak implies, Code Review is an appropriate place to ask "Should it work this way?" and "How can this do X better?" questions about working code. And the code in the question is presented as working (just possibly non-ideal, or that was the author's concern).
On the other hand, the post was probably completely on-topic and constructive on Stack Overflow. While answers would have been subjective (as daramarak says), they could easily be supported by evidence and narrowly scoped.
Plenty of perfectly good questions on Stack Overflow have multiple perfectly good answers that disagree with each other. It's certainly true that subjective questions tend to be less appropriate on Stack Overflow than on Programmers or Code Review, but Good Subjective, Bad Subjective still applies, and this question seems to fall on the "good" side (by the six criteria listed there).
If a question is clear, on-topic, and constructive for Stack Overflow, then unless the OP requests migration, it should not be migrated to another site even if it would probably have been better to ask it there. Being a clearer fit somewhere else does not make it off-topic for SO.