Earlier quoted context omitted.
By switching to their own hardware, they become NVidia's competitors.
That's kind of arguing that people cooking at home are restaurants' competitors. There's a kernel of truth in it, but if I was McDonalds I'd care a lot more about what KFC is doing than the market trends of cast iron pans.
90% of Nvidia's revenue is from datacenters.
If the datacenters stop buying Nvidia's products, and use their own hardware instead, then Nvidia loses 90% of its revenue.