On iOS you can’t side-load apps. You can only run native code if it has been manually approved by Apple and acquired via their special App Store. There are a handful of exceptions but they have their own limitations.
Android is at worst as closed as Windows, but not more. Some perspective is called for.
And it's always been possible if you have a developer account (and I might be wrong, because I quit mobile development to focus on cloud but I think you don't even need a paid developer account to build and load an app from source code anymore -- but I could be wrong).
It's problematic at minimum.
Meh. Load LineageOS or GrapheneOS onto a phone without Google Apps, you won't get the same map functionality or on-device voice-to-text. But it's still android and 100% open source.
It sucks they are adding cool stuff to the GApps side, but pretending like this makes the OS as closed-source as iOS is absurd.