It depends on what you mean by "serious". Software development is very much a free-for-all as far as backgrounds go. It attracts people from all disciplines. There are NO CREDENTIALS needed to be called a "software engineer", and it will remain this way for the foreseeable future. Every organization that develops software has its own rules and standards for how they staff their teams, usually these rules are flexible to almost non-existent. They hire based on whether they think someone can do the work based on some loose evaluation criteria.
But, yeah, a mid-to-late career-changer with a successful business background as highly paid executive is going to raise eyebrows if they go after some software development job-- not because of technical chops (assuming they still got it), but more "wtf are you thinking?" considerations. These people would normally just self-fund their own software projects, or partner with someone in exchange for having some fund with code (it happens).
CEO demographics vary wildly. The article just cherry-picks the top ten market cap companies, which today, happen to dominated by software companies. Most of the CEO's have been SE's? So what? That means almost nothing. Are these 10 companies supposed to be profoundly representative of the world of work in some way?