This is very false. Fuchsia is built around the idea of making it easy to port software. It just doesn't attempt to achieve it by providing a POSIX interface. POSIX doesn't specify the vast majority of interfaces a modern OS needs to define anyways. For instance you won't see POSIX interfaces that tell you that provide you a signal for when memory on the system is low and you should free up memory.
Instead Fuchsia tries to target porting runtimes which applications are built against. For instance, porting chrome as a runtime unlocks web applications. Porting flutter unlocks flutter applications. Porting Android as a runtime is challenging but still achievable. It is not necessary to rewrite all of Android to accomplish this either.
At the extreme, it's possible to simply implement runtime support by implementing various virtio interfaces and running a full OS in a VM, similar to how other OS like ChromeOS and Windows achieve Android app support today. Fuchsia is written in a way that you don't need to do that, but it's always an option as well.
Instead Fuchsia tries to target porting runtimes which applications are built against. For instance, porting chrome as a runtime unlocks web applications. Porting flutter unlocks flutter applications. Porting Android as a runtime is challenging but still achievable. It is not necessary to rewrite all of Android to accomplish this either.
At the extreme, it's possible to simply implement runtime support by implementing various virtio interfaces and running a full OS in a VM, similar to how other OS like ChromeOS and Windows achieve Android app support today. Fuchsia is written in a way that you don't need to do that, but it's always an option as well.