People who build aircraft are not people who design aircraft.

Moreover, the point of the comment isn't about the titles assigned to these people by someone external: it's about the actual skills they have that would cause you to assign such titles to them.

If you know people who can design new aircraft, then they are, by definition, aircraft engineers. They may have a job as an aircraft mechanic, but that's besides the point. That doesn't mean the average aircraft engineer is capable of designing aircraft.