If we are talking about platforms and not codebases there are waaay more than two.
iOS (App store)
Android (Google Play)
FireOS (Amazon App Store)
LineageOS (Fdroid)
One UI (Galaxy Store)
To name a few. These all have distinct and thriving marketplaces for apps with decent market share (some can overlap). The code base they use is irrelevant for this context.
How many of those are installed on more than 1% of mobile devices (however broadly construed)? I think the answer is two. Amazon has like 10% of the tablet market share, but the tablet market is like 1/20th the size of the phone market.
Just to demonstrate that this is the variable that matters, consider the case where there are exactly three devices that have each of the other marketplaces installed on them, respectively. Would they present a meaningful competitive alternative to the big two?
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.
If we are talking about platforms and not codebases there are waaay more than two.
iOS (App store)
Android (Google Play)
FireOS (Amazon App Store)
LineageOS (Fdroid)
One UI (Galaxy Store)
To name a few. These all have distinct and thriving marketplaces for apps with decent market share (some can overlap). The code base they use is irrelevant for this context.