Most versions of Android are also locked down and completely proprietary
Your claim that versions of Android are completely proprietary is obviously false: every version of Android uses a lot of other open source software, and runs on Linux, which is GPL.
You may be able to sideload apps, but you're still limited to the few APIs that they allow you to use, while these corporations still have the keys to the root account. Even if you own the phone, you can't get around these controls.
even if the low-level bare-bones distribution of android is free software, the vendors almost always tack on garbage that makes it completely compromised. It's like "I may be in a prison but the fair-trade handcuffs are made by a B corporation so I'm happy"