There can be many explanations for this, but the simplest is that when you have a tool in your tool chest, you tend to use it, and when you use a tool, you see opportunities to use it everywhere. Whereas if you don't have a tool, you don't see the opportunities it affords, so you don't know what you're missing.
Even if my conjecture is true, this doesn't mean that mathematics is more important than some other experience a programmer might bring to their career, so I am not suggesting that the author is wrong when suggesting it isn't important. Perhaps it's useful but less important than some other skills.
I'm not a mathematician, so I don't know, it's all blub to me.