It looks like this aims to be a replacement for people want to use an Android-based OS but don't want to rely on Google services, normally for reasons of philosophical objection to big tech's de facto surveillance capabilities. So presumably users of GrapheneOS, CalyxOS, LineageOS, etc.

Also since Google is blocked in China, Firebase Cloud Messaging doesn't work there. Android has something like a 75% market share in that market. Some vendors have their own offerings (e.g. Xiaomi MiPush, Huawei PushKit) etc, I could buy that a popular centralised solution would be more attractive than doing N integrations.