One really oughtn't downplay their research skills like this, rather they should wear them as a badge of honor. Downplaying them leeches away one's power.

Ph.D. researchers may potentially go deeper into sources, but this is only a function of time and available attention.

This sort of debate also plays out in spaces like writing computer code. The broader industry determines who is and isn't a "coder", but this is only a means of creating power structures that determine who has power and who doesn't or who is part of the conversation and who isn't.

Don't let Maggie fool you here, she is definitely part of this conversation.

What areas of work over time does this pattern of level of experience not apply to?

There is definitely a level of minimal literacy at which one could be considered a reader, but there is no distinction between amateur reader and professional reader the way there might be between an "amateur researcher" and a full time "academic researcher".

Other examples of this? Video game playing?

THIS IS A BOLD STATEMENT. "Amateurs"? I think that music should be accessible and allowed to be experienced by anyone (can be experienced in diverse ways). If we contextualize this statement. We are all amateurs of another's portrayal of music. We cant ever be experts of all music (unless we get to the point that all music is under the same structure.)