This is actually not a very good estimate. It has many inaccuracies, and those inaccuracies are harder to nail down than the original problem, so you made it harder on yourself.

Some questions about your result: How many piano tuners work at any of those piano tuner companies? How many piano tuner companies are not on yelp? How many piano tuners are employees of instrument stores instead of working freelance/for a tuner company?

Those are all harder to answer without industry information than the simple deductions the handout makes from common sense/knowledge.

I have the feeling it is important that people understand the limits of technology. Even though we like to say something doesn't exist if it isn't on Google, things very often do irregardless.