There are different aspects.
As has been said, the first one is that with a Pixel, the company manufacturing the phone and providing the OS including all standard apps is just Google. As all parties involved will try to spy on you (generally), having just one involved beats multiple. If you’re using Android, Google will usually be involved anyways, so a pixel is the only device where it’s just Google instead of Google + another party. You get the stock Android experience, without anything forced on top.
Also, Pixels are relatively cheap, come with long update support and quick security updates. Generally, Google is one of the better companies when it comes to security. From the hardware side, Pixels are some of the most secure devices.
I don’t know whether iOS on an iPhone or Android on a Pixel is more private out of the box. From what I know, the difference isn’t too big. However, Android is more secure, as evidenced by higher exploit prices for Android than for iOS.
And Android, especially on a Pixel, gives the user a lot more options to make the phone more private. Not only can you install F-Droid, Accrescent or Obtainium — you can also just completely get rid of Google by opting for an AOSP custom ROM. The best option for privacy, security and usability here is GrapheneOS, which only works on Pixel because they’re the only devices meeting their security standards.
So Pixels at worst are a cheap, long supported, secure Android device with less parties spying on you. At best, they can be fully degoogled, alternatively almost degoogled with great usability (Sandboxed Google Play), extremely secure devices. No matter how much privacy/security you want to achieve and how much resources you want to put into that, for everyone from “I have nothing to hide” to Edward Snowden, Pixels are a great choice. In my opinion, the best.
That’s the funniest thing I read today, and it’s made even better by being 100% true.