Again, untrue. I don't know where you're getting the idea that it's only just Play Services. For a long time now, Google has been delivering updates to various Android system components via that updater. The whole point of it was to deliver critical updates to certain system components outside of the usual channel, so that you didn't need to depend on your manufacturer or carrier to issue a full OS update. It makes no sense to limit this just to Pixel phones, that defeats the whole purpose of these updates.

Your very own link shows all the changelog for all the different components that are getting updated, not just Play Services. And this is not just limited to Pixel phones (what's your source for that?)

From the link in your link:

Google System updates give you new and useful features that make your Android devices more secure and reliable. It includes updates to the Android operating system and key system services provided by Google, Google Play Store, and Google Play services. Google System updates are available for all Google-certified Android devices

https://support.google.com/product-documentation/answer/11462338?sjid=2146626811830280084-AP

Samsung devices are certified, hence they too receive these updates. If these updates were limited just to Pixel phones, they'd explicitly specify that on the page you linked.