Google has very direct control over Google Play internal services which are effectively system-level services on android, and collects at least as much data on those users as Apple does on theirs.
The 1% of users running some custom Android build aren't really relevant here, Google scrapes up plenty.
> That is, almost two order of magnitude is uploaded by Android (3.6MB) than by iOS at startup, while Android downloads around a third (952MB) of the data of iOS (2.6GB).
The paper goes on into a lot more detail of tracking what identifiers are sent when and how often.
The 1% of users running some custom Android build aren't really relevant here, Google scrapes up plenty.