Yes and no, I think that depends how deep you want to go. For your standard "show and update some content and make it look pretty" type app, I can tell you from both my own and others experience you don't really need much familiarity with the native platforms, other than how to use build tools etc. which are well detailed online.

If you want to go deeper or have more specialised requirements, then you might need a deeper understanding of the native platforms though.

I can speak for Qt, Ionic and Cordova.

Beyond "Hello World" all of them required me to get my hands dirty, at which point I was better off just doing C++ with native views, standard out of the box tooling available across all SDKs.

Nowadays I rather go for PWAs, or plain mobile Web, unless it is something very special that requires OS APIs or hardware access.

I've worked power-user type applications that are mainly keyboard driven, and those are pretty easy to make cross platform with a single custom codebase that has a platform-specific graphics backend (OpenGL covers many platforms) and minimal platform code. Let's say < 300 lines of boilerplate per platform + 600 lines for OpenGL + 2-5K lines for (shared) UI controls.

Yeah, but that kind of fits what I was describing, you are using standard SDK tooling, and it looks to me it being game engine like.

I wasn't disagreeing.

Neither was I. :)