If I understand the notifications flow correctly: a device (typically mobile phone) will get a "registration" with a "push url" from its platform/provider (eg android/google or iphone/apple), which it hands to an application server (eg IMAP server), which then uses that to send notifications to the push url, "authenticating" with vapid. And due to vapid, no special account/preregistration is required for the application server to talk to the provider, hence decentralized (good!). The platform/provider behind the push url makes the notification appear at the device. It is quite a complicated way to get a message from server to device...
The post says it is about Android. But I'm online searches turns up documents about VAPID at apple/ios. Will it work there too? What about other platforms (eg non-google android-likes)?