That's why KRAD's saying the author doesn't matter of a tie-in novel; it's all about grabbing that non-regular reader in the consumer majority. That's the way to expand your sales beyond the minority fanbase and have a big success.
Rather, it's about the difference between the individual and the aggregate. He's not saying the author doesn't matter to any individual buyer, because he's not talking about buying decisions on an individual level. He's talking about the overall sales figures and profits seen by the publisher. When it comes to those figures, who's writing a Trek novel has much less impact on the sales and profits than what series or characters are featured in the novel.