Having spent a couple of years in M&A and capital markets, I can attest that this is absolutely true.
In most cases, most top M&A buldge bracket firms and even top M&A boutiques, will look to work on sell-side deals that are $500mm+. Probability of a successful close is so low and opportunity cost is high (process is long and it's a ton of work), it rarely makes sense to look at something unless the fee opportunity is in millions.
Simply put, $10mm deal doesn't move the needle for top firms. There are exceptions, of course, primarily for relationships and future business potential, for example when spinning off a smaller division of a larger business or when dealing with financial sponsors.
"I just believe Kan when he says that the ones who are actually worth their fee won't work on a 10MM deal."
That really sounds a bit like "anyone good works for XYZ" (pick some of the usual suspects) or "all top lawyers work for big NYC law firms and have made partner" (or similar .... you get the point). Or the local business man in a no name town, that nobody has ever heard of, couldn't possibly be as sharp as a guy operating where all the action is. Why would anyone be in Bentonville if they can be in NYC or Chicago?
You are making an association between what a top firm does and how they evaluate opportunities and what an individual who happens to work somewhere else (for some particular reason perhaps he or she can't move on to "bigger and better things") and what their actual skill level is which are two different things.
As a generality of course it can be true and a shortcut. But making an assumption that someone couldn't possibly provide value (assuming they don't harm a deal of course) I don't agree with.
