it’s only open in words, the android phone you purchase from oems contains plenty of proprietary stuff

The core of Android is completely open-source. But yes a typical device has a ton of proprietary drivers layered on top of it, along with a bunch of proprietary Google apps and frameworks.

That’s still way better than nothing: sure the drivers you can’t do much about them, but you can still build a fully functional de-Google ROM if you want. I see it kind of like installing the NVIDIA drivers on Linux: not ideal, but it doesn’t affect my ability to modify the Linux kernel or any other part of the operating system.

It’s not like PCs aren’t loaded with proprietary firmware either. We may have open-source kernel drivers, they still upload proprietary firmware to the device for your WiFi and GPU to work. Very few PCs can be corebooted.

in some cases like play integrity, an open alternative doesn’t exist on top of that

That’s not completely true. The APIs for it are completely open to any app, but apps that check Play Integrity specifically are also doing so specifically to check for Google-approved ROMs. Apps from the Samsung Store can use Knox instead to do a similar thing.

If you want to use the TEE and make sure your app only runs on official GrapheneOS or LineageOS builds, you can. It’s just, nobody does that because why would anyone do that. But if you have an application that wants it, idk you somehow have corporate devices that should run your custom AOSP build and prevent rooting or flashing to run your custom proprietary app, you totally can.