@ Alice
Certification can be tricky. Any certification for that matter There are various levels of certifications to teach English as a foreign language. Some people have a master's in TESOL (more in-depth training), while others are fresh out of college and have a certificate which can be obtained by studying/training for less than a year.
Needless to say, certification does not guarantee good quality of teaching. Teachers who are teaching English for more than 10 years without certification may be better than many teachers who only have certification and no actual experience.
I highly doubt ALL teachers with certification can edit professional writing appropriately as you said because correcting documents used in professional settings needs a perspective gained from hands-on experience as an employee to understand the context. . They may be able to correct grammatical mistakes, but not be able to edit to suit the purpose of writing. Can they revise a marketing press release? Can they understand IR documents without a degree of economics/finance or real experience in the field? Editing involves the right word choice, tone, style and many other aspects.
That's why I think knowing specialities is important when it comes to finding teachers.
Maybe italki is only for students who want to casually improve languages, so it does not cater to users who wish to improve specific areas or for specific purposes.
I'm very much interested in hearing opinions from teachers.