The author defined a professional as someone whose work you have to take on faith. Not being doctors or lawyers ourselves, we're usually unable to evaluate the quality of legal advice or medical care we're receiving. The best we can do is seek another opinion.
The author drew this line because he feels it necessitates the things we expect from a profession: shared ethics and a rigorous certification process. Professions require would-be professionals to jump through a number of hoops to make sure that all who deal with these professionals receive some minimum level of service from someone with a minimum level of competence.
Have software developers arrived at this point? Can someone check the quality of what they receive from a developer without being a developer? If they can't, will this always be true?