I don't think there's a definition like that anywhere. Only if you define 'amateur' as some who doesn't really know the trade. We should remember that 'amateur' actually means "lover of", i.e. that an amateur is simply someone doing it because they love to (as opposed to "just" because they're paid to do it). It doesn't say anything about the level.

Yes, they would be. Amateur developer in this context can be interchangeable with open source developer for instance, programming as a hobby with others or just for yourself. It's just a title. You can be both a professional sw dev and an unpaid opensource dev at the same time. Likewise, you can be both an amateur HAM operator and a professional RF engineer at the same time, exclusive of each other.

So a professional football player who after his career joins an amateur team for recreational football should not be regarded as an amateur?