That is but one definition and it's American-English, not English. The more complete definition from the OED says:
A. adj.
†I. Senses relating to a profession or vow.
1. Relating to or marking the occasion of entrance into a religious order. Cf. profession-ring n. at profession n. Compounds. Obsolete. rare.
2. Of or relating to a profession or declaration; that is avowedly (but sometimes falsely) the thing specified; professed. Obsolete.
II. Senses relating to or derived from (the conduct of) a profession or occupation.
3.
a. Of a person or persons: that engages in a specified occupation or activity for money or as a means of earning a living, rather than as a pastime. Contrasted with amateur.
Sometimes applied disparagingly to a person who makes a trade or profession of something usually associated with higher motives.
b. Of an event, activity, occupation, etc. (now esp. a sport): undertaken or engaged in for money; engaged in by professionals (as distinct from non-professionals or amateurs).
c. In humorous or derogatory use. Of a person: habitually making a feature of a particular activity or attribute, esp. one that is generally regarded with disfavour; inveterate.
4. Of, belonging to, or proper to a profession.
a. Relating to, connected with, or befitting a (particular) profession or calling; preliminary or necessary to the practice of a profession.
professional examination.
b. Engaged in a profession, esp. one requiring special skill or training; belonging to the professional classes.
c. Characteristic of or suitable for a professional person; (now esp. of equipment) of a type used by professionals.
d. That has or displays the skill, knowledge, experience, standards, or expertise of a professional; competent, efficient.
5. That has knowledge of the theoretical or scientific parts of a trade or occupation, as distinct from its practical or mechanical aspects; that raises a trade to a learned profession. Now rare except as merged with senses at A. 4.
6. Physiology. Designating or relating to a cell which is specialized for the ingestion of particles by phagocytosis, such as a macrophage or neutrophil (as distinct from a cell capable of occasional phagocytosis).
III. Senses related to the office of professor.
7. = professorial adj.Now rare.
B. n.
1.
a. A person who engages in a specified activity, especially a sport, as a paid occupation. Frequently opposed to amateur.
b. Chiefly colloquial. A prostitute.
2. A person engaged in a profession, esp. one requiring special skill or training; a professional person, or a member of the professional classes.
3. Scottish University slang. Short for ‘professional examination’
4. A person who does something with a high level of competence, commitment, or expertise.
-----
So the use in this case can be construed to be entirely correct, c.f. Sense II §4 and Sense III §2 and §4