The phrasing of the reason is unfortunate, IMO. It makes sense to migrate if a question is better suited to MSE; it doesn't necessarily make sense to migrate just because the question can be answered on MSE. In what follows, let me pretend that the wording "can be answered" has been changed to "better suited" since that may have been what was intended anyway.
Suppose that a question that is better suited to MSE gets asked on MO and is well-received. Does that mean it shouldn't be migrated? I'm not sure. Whether the question is well-received is an a posteriori criterion and I think that in general, we should be using a priori criteria to judge which site is more appropriate. Otherwise it tempts people to try to "game the system" and try their luck posting to a site that they kind of know is the wrong site.
For the particular question under discussion, I think the point is that it's not clear that MSE is actually the better site. The question does have some flaws; as someone has pointed out, the asker should probably have done some Googling first, but these flaws just mean that the question should be improved (regardless of which site it is posted to), and don't really address the issue of which site is better. Andrés E. Caicedo's comment indicates that some version of the question could have been better suited to MO, but I'm not sure that the question as asked is better suited to MO.
I agree that migration can be disruptive and annoying. So I'd propose the following policy: if there is doubt about whether MO or MSE is more suitable, then leave it on MO; but if it's fairly clear that MSE is more suitable, then go ahead and migrate it even if the question is "well-received." (Of course, if the question has flaws, then those should be addressed, probably before any decision about migration is made.)