This seems like the right answer. An "expert" should be someone whose field of expertise is in the subject being presented to the court. The opposing counsel can (and should) challenge it when appropriate. A PhD in medicine or PE cert is a pretty good bona-fides to presenting a person as an expert to the court, but it doesn't mean that some amateur naturalist whose spent 20 years studying the population of butterflies in the Sierra foothills couldn't be considered an expert in that specific topic of discussion either. Just that their scope might be more limited than the Phd.
California law doesn't seem to make the sort of distinction either, for example:
720. (a) A person is qualified to testify as an expert if he has
special knowledge, skill, experience, training, or education
sufficient to qualify him as an expert on the subject to which his
testimony relates. Against the objection of a party, such special
knowledge, skill, experience, training, or education must be shown
before the witness may testify as an expert.
(b) A witness' special knowledge, skill, experience, training, or
education may be shown by any otherwise admissible evidence,
including his own testimony.
721. (a) Subject to subdivision (b), a witness testifying as an
expert may be cross-examined to the same extent as any other witness
and, in addition, may be fully cross-examined as to (1) his or her
qualifications, (2) the subject to which his or her expert testimony
relates, and (3) the matter upon which his or her opinion is based
and the reasons for his or her opinion.
California law doesn't seem to make the sort of distinction either, for example:
720. (a) A person is qualified to testify as an expert if he has special knowledge, skill, experience, training, or education sufficient to qualify him as an expert on the subject to which his testimony relates. Against the objection of a party, such special knowledge, skill, experience, training, or education must be shown before the witness may testify as an expert. (b) A witness' special knowledge, skill, experience, training, or education may be shown by any otherwise admissible evidence, including his own testimony.
721. (a) Subject to subdivision (b), a witness testifying as an expert may be cross-examined to the same extent as any other witness and, in addition, may be fully cross-examined as to (1) his or her qualifications, (2) the subject to which his or her expert testimony relates, and (3) the matter upon which his or her opinion is based and the reasons for his or her opinion.