The author explains how analyzing and presenting data was worth millions of dollars. The author documents how a senior executive instructed him to write excel. It is clearly their core business. Also, and this comes back to the fantasy/denial/wishful-thinking aspect here, neither I nor the law says core business. That's a word that you added. If you did it as part of your job, then it is, by definition, part of their business.

I am also nearly 100% certain we can look back at this comment in 20 years and find nothing happened, but only because nobody will take this code and make a billion dollar business. If they did, I guarantee there would be a law suit.