Hey, author here. You're asking a couple questions, which I'll try to unpack.

You're right to be suspicious that Cloudflare CDN servers have the right time. I tried to call out that concern in the post. Cloudflare operates time services, but their CDN is not a time service and wouldn't be operated under the same controls. How accurate those servers are is an open question.

When writing the post I observed NTP client logs, a GPS-based clock, and the CDN-based clock report time that agreed within their stated error bounds. I've also seen NTP-synchronized and mobile network synchronized system clocks that fell outside the estimates of those other clocks. So it's an "odd one out" situation.

But I think you are asking the right questions!