I have often seen academics and science communicators who are positioned as subject matter experts in a field or topic but haven't been directly involved with doing research since their Ph.D. or postdoctoral training years ago. Or they had their training in some other topic in the field ages ago.
Does the ability to read and assimilate information from a paper (irrespective of having practical training) enough to be called an expert? Or is it necessary to be an active researcher in the subject to be an expert?
For clarifying my question:
I have an undergrad, Master's and a Ph.D. degree in materials science with a focus in aerospace materials but am now reading papers on battery and solar cells (which are completely unrelated topics in materials science). Can I be called an expert in battery and solar cells if I have read current research papers, got some theoretical understanding and can have a constructive discussion on the topic? I can distinguish between a good research and a poor one. But don't have practical experience or research output in the topics.
A person is an expert if someone calls her or him a expert. It could even be that someone declares herself or himself an expert. Be careful when doing so: it works surprisingly often for a general audience, but at the same time makes you look silly within that field.
In short, expert is not a protected title, so it could mean anything.