As a sibling commenter said, those proprietary apps have a server part that sends notifications to Google's server. If UnifiedPush were to gain enough traction

As a sibling commenter said, those proprietary apps have a server part that sends notifications to Google's server. If UnifiedPush were to gain enough traction (say it would be adopted on FireOS, HuaweiOS and whatever platform don't use Google services), those apps would likely start to support it directly. Another possible avenue is to add UnifiedPush support to push libraries used by these apps both server and client-side. Multiple libraries (Google FCM included, I linked at least two others from Amazon and Huawei in another comment here) abstract push notifications away so the developer gets a single server to talk to, and a unified API across iOS, Android, Web, etc. If these libraries started supporting UnifiedPush, adoption could increase a lot, without involving any developer effort for these proprietary apps.