Beyond that though, there's very little dependence on Google for apps. The web store opens in a browser window and (if someone were to create one) could easily go to a different store.
Likewise, Android-compatible Chromebooks can use Aptoide same as any other compatible Android device (although Android should be looked at too IMHO - I should at least have the option to say "I trust X app store in addition to Google Play" without removing un-named sources protection altogether. Likewise iPhone.).
That's in Android 8.0: http://www.androidpolice.com/2017/03/21/android-o-feature-sp...
Only if you're looking to find fault. The analogy to cars works fine but only if you compare like for like.
There's a direct analogy to fueling and that's charging, which can be done from any electric source as long as you have the right connector (like cars).
Apps are more like car modifications. For most modern cars this is very hard to do without using the manufacturers own fitters or preferred suppliers. Some modifications are possible without their consent but they'd definitely void any warranty and they're not exactly encouraged.
BTW I'm not arguing that this is fine. Just that cars are as much, if not more, walled gardens than phones.
To be honest, given the economics of most consumer goods, it's hard to find anything that encourages non-manufacturer modification or repair. If courts were to really start fighting this, they need to fight pretty much everyone.
Sounds like the Nespresso business model too.
Android is actually more restrictive, since you can distribute software outside the Google Play Store but hardly anyone does.