This isn’t an entirely new concept because there are some PC games whose downloaders let you play the game even before everything is finished downloading and installing. This is usually accomplished by where the critical files needed to run the game are downloaded first or given priority, and that other not-so-essential files are downloaded later.
This means that you could play the game even before all of its contents are downloaded, and by the time the download is complete you would probably be too immersed to have noticed. Google has attempted something similar in the past with its instant apps, so while this isn’t quite the same, it’s along a similar vein where Google is trying to give users access to apps as quickly as possible.
This is a feature that is expected to be available come the Android 12 update, so it might be worth keeping an eye out for.