No that's not true at all. All the photons from emission Nebula are contained within emission lines and as such a narrower the filter the better irrespective of if its OSC or mono. The filter will always let all the nebula light through but will block increasing amounts of broad band light as it gets narrower, that includes both light pollution and star light so smaller stars are a secondary bonus. But for this reason you would not use a narrow band filter for galaxy imaging, unless its to image Ha regions within a galaxy and add them to a broad band image.
The only slight gocha used to be with older DSLRs, they would have a hard time focusing / finding a target with very narrow filters as they could not see a star in live view mode. That is absolutely not an issue for a camera like your 294 which is more than sufficiently sensitive.
Would love to know who is feeding you that information.
Adam