Are the contents of push notifications not encrypted? Or are we talking about payloads rather than transport?
They mention metadata in the article. Imagine sending a message to a Signal account at time X, then asking Apple a list of all users that received a Signal notification at that specific time.
I.e, the push notification itself contains little to nothing in terms of data/metadata.
You can also of course decrypt a notification by shipping an extension to do so, and maybe Signal does - it’s been awhile since I poked around it. I’d just be surprised if the Signal team didn’t analyze the issue to death and find the gaps.