>> What about related or synonymous keywords?

This post does not go into detail on the full approach for measuring a market. That's detailed in my book and way too long to include in a blog post.

This post simply talks about the 4 most common mistakes people make when doing this kind of research. Nowhere did I indicate your entire market research depends on this single keyword. That would be stupid.

>>What about other traffic sources like banner ad media buys?

There are at least 50 other traffic sources: Twitter, blogging, podcasting, other search engines, magazine ads, TV ads, radio ads, cold calls, direct mail, Facebook pages, Facebook ads...I could go on and on.

None of them offer data that allows you to easily perform market research. Also, unless you are funded many of them are completely out of reach.

Again, Google is not the only source of traffic but it's an easy way to measure market demand.