FWIW, the certification thing is probably a good thing to do if you plan on making a career of teaching tennis. People like the pieces of paper. It does show at least a certain level of knowledge. I'd treat it as a necessary but not sufficient condition.
You also have to be a student of this game. It's constantly changing and good teachers keep up with the change. Good teachers have methods of teaching as well. It's one thing to know what the final stroke should look like, it's another thing to know how to bring someone along in steps so that they can get there.
It's also important to know the variations that you'll come across. What's the difference between teaching a high level Western forehand verses a high level strong Eastern forehand? Weak teachers just try to force everyone into the same mold. Strong teachers understand the differences in the game and work with their student's preferences. A good teacher also knows when to push a student to change something that won't work (a high level player that wants to volley with baseline grips for instance).
The certifications will help you get jobs and send a message of professionalism. The lifetime of knowledge are what's going to help you be successful at your job.