You could probably do it with Cloudflare for a similar price, but that's not what this article is about. The article says that their Cloudflare (not Argo) bill is only $40/month, and that's only because they have two domains. It's $20/month per domain.
The $400/month figure isn't just for static file hosting. It includes their API, database, and global CDN. They make a point to say that low-latency, global CDN is a priority for them, which is why they're paying extra for Cloudflare Argo.
I'm not sure why everyone is comparing what they're doing to a single fileserver hosted somewhere, serving up static files without regard to global latency. It's apples and oranges.