Samsung's market is installed on a substantial percentage of phones but almost no one on any of those devices uses anything besides the regular Google Play store. So while you technically answered his question in the larger conversation Samsung's app store isn't a competitive alternative.
I couldn't find robust stats on Amazon's market share overall but I did find that they sold 5.3% of all tablets in Q3 2019. So if its true that the phone market is 20 times larger than that it would mean its unlikely that The Amazon App store is installed on 1% of all mobile devices.
So users do use playstore in the presence of the other store. Doesn't that contradict the argument that they are poor non-tech walled-gardened who cannot choose? This entire farce is about developers who want to control platforms they do not own, on devices they do not own, with users who do not even want to deal with them under rules other than playstore ones.
I am skeptical that Amazon has 1% market share. I found a datum that Samsung has like 25M MAU in the US, but I doubt that they do more than 1% of the total app transactions in the US. Maybe in Korea.
4/5 in the examples I gave.
Google, Apple, Samsung and Amazon all have decent market share.