Google's app store is not exclusive.
But then again, Google paid billions to manufacturers so their store was the only option preinstalled in their devices, which meant that the vast majority of users would just stick with it.
Google positioned itself so it is the de facto default for pretty much every Android phone.
And if having an app store that doesn't require an account is actually somehow desireable, is your suggestion that Google somehow strong-armed Samsung into requiring an account rather than going with the ostensibly superior option of not requiring accounts? Or is the suggestion that the store should be using some other account than a Samsung account, but they weren't allowed to do that.
My point here is that it does not matter that a device comes with one, two, or multiple app stores, if the whole start up flow steers users towards a specific one.