Perhaps it would be better to say: "a salesperson is unlikely to maximize their potential selling something that they don't believe actually helps the customer"
There are great salespeople the world over selling products they don't believe in — after all, great products tend to sell themselves — but you can bet your bottom dollar they'd be doing an even better job if they did believe in the product.
Based on the sales people I have known, most would pretty much say anything to close a deal. Though the really good ones seem to be able to convince themselves that the product they are selling really is awesome. As soon as they stop selling that product they go back to their old opinions on it being a POS.
I guess some people might manage to bullshit themselves into thinking the product is awesome when it's not... then again, even in those cases, there is presumably some element of the product that is awesome, otherwise they couldn't believe such a thing?
Self-delusion is a powerful force, but smart people need more than that to achieve any kind of lasting result.
From what I understand they often start with some sort of brainwashing, they are also taught weird beliefs like "every time a customer says no the odds of the next customer saying yes will increase".
I guess it will depend on what the product being sold is, enterprise software is probably somewhat different to timeshares or double glazing.
It is worth bearing in mind though that some of the best salesmen throughout history have probably been people who were knowingly selling scams.
An assertion of fact can be right or wrong, but it can't be a fallacy. He's saying that not believing in your product will cause you to fail to make sales, not that it will disqualify you from greatness by definition.
There are great salespeople the world over selling products they don't believe in — after all, great products tend to sell themselves — but you can bet your bottom dollar they'd be doing an even better job if they did believe in the product.