As you can observe, DocType has changed to a js code instead of JSON. It is not designed to be compatible with each other.
Yes. FrappeJS will (under active development) make it easy to develop cross-platform apps. Developers have to be familiar with one language and framework (JS) instead of many, Java/Kotlin, Objective C/Swift.
FrappeJS plans to make electron app development, ionic app development, or any nodejs app development “batteries included”
FrappeJS is new and actively developed for cross-platform desktop, mobile apps.
It does not have web development features that are required to develop comprehensive server side apps like ERPNext.
If comprehensive proven back-end is required for enterprise with hosting and technology capabilities? Go to frappe python (new apps can start with python 3)
If you are developing offline first apps, web rtc apps or serverless apps, go for FrappeJS.
All start as experiments, PoC, MVP, bare-bones and then develop into usable software.
Frappe/ERPNext : Stable proven, used by many, Enterprise level solution, batteries included web apps.
FrappeJS (and set of apps it will develop) : For Single user or small group of users, cases where server setup is difficult or offline app is necessary.
FrappeKt : Not under active development, can be used by developers to build their own apps (does not cater to need of being cross-platform as it is Android only)