Wow they buried the lede; I'm glad you highlighted this. This is going to be an interesting situation for some Safari users. "Why did your app delete my data?" "If you don't like that you should use Chrome."

Of course Google could do this too, if they had a reason, even if only downstream from Chromium. It's just a commercial decision. Apple have decided they don't want their users to have usable anonymous web apps. Of course, since they don't support beforeinstallprompt, we already know they don't want their users to have web apps, period. Gotta get that sweet 30% cut!