Sorry that’s absolutely not the case. They’ve created “locked down” experience so that is serviceable and harder to break but you’re absolutely free to install arch, ubuntu, whatever and put Steam on top of it. You can install Windows even.
You don’t have to choose “all or nothing”. You are not being locked in an ecosystem like you do with the consoles Steam Machine aims to compete with.
It is locked down in exactly the same way you can go through a bunch of steps to unlock any other device. SteamOS is locked by default. you must run a series of scripts found on various forums to unlock it.
Windows is broken on steamdeck. the only OS that works properly is SteamOS for obvious reasons.
You don’t have to choose “all or nothing”. You are not being locked in an ecosystem like you do with the consoles Steam Machine aims to compete with.