Portability of software is a language problem. Some languages make it extremely easy. Others, like C and Go, make it difficult to be correct across all platforms.
If you have the same APIs available (OpenGL in this case) C is the most portable language there is. Which proves it's not a language issue, it's a platform issue.
In fact the owners of the platforms care so little about (indeed actively oppose) compatibility between them that increasingly they make their higher-level APIs only available to a single language so you can't even share the parts of your code that aren't dependent on their APIs. Unless of course you drop into the one language that works everywhere and which they have no choice but to support: C (or something else that compiles natively and can present a C API like C++, Rust or Kotlin native).
Java's "write once run everywhere" has become a sad joke.
I've been tinkering with Rust bindings for Apple Core Audio. AUGraph, which has a C interface, was deprecated a few years ago. Its replacement, AVAudioEngine, has an Objective-C/Swift API. :(
There are a lot of generalized cross-platform libraries for guis, audio engines, etc, which attempt to abstract over the differences between native libraries. While noble, these are always going to be very limited and difficult because the library interfaces may make radically different assumptions and have very different high-level structures.
To me it seems as though the best strategy to help programmers who want to do cross-platform development similar to you is to focus on providing complete bindings for the platform-specific native libraries (for me the bindings language is Rust but it could theoretically be another one like Go). Once those exist, it expands the amount of code you write that can be shared. It also becomes more straightforward to write cross-platform abstraction libraries.
However, my sense is that the platform vendors are heading towards a future where C ABIs are eschewed and where sharing code will be utterly impossible.
Had this not been the case and even your C code would have been touched.
if you wrote your iOS app in Xamarin, u could today run it on .NET6 pretty much unchanged. I maintain multiple store apps where i share 90% of the code across all platforms (iOS/Android/Win/macOS) that have extensive integrations with the OS. I can even drop down to native obj-c, java, win32, macos apis as required using the same syntax as any other library thanks to the code-gen MS does. I use powershell for scripting. Dont need a full app? I can have native islands as well or be an island in another native App.
I think this is one of those statements that sounds good at face value but doesn't stand up to scrutiny.