Whatever job we think require the very best in the field are often not performed by the very best. You'd think a book on topic X is written by an expert in X, but the truth is that it only takes enough skill to fool the average reader to sell books.

TV chefs? Mainly have to be good at talking.

People who write libraries, compilers, OS kernels and so forth? All sorts.

So never ever assume that someone is an expert just because they do something "expertly".