This is solved by knowing what is stored is private browsing data, which should be marked anyways. Safari just isn't that well thought out of a browser.
Safari is just a browser that has made a different set of tradeoffs.
Who defined it that way? And how do you define 'exit', 'data' and 'exist'?
Leveraging native APIs means less code (fewer bugs), less disk usage, less RAM usage, etc.
>> The point of private browsing is to make it so when that window exits, that data doesn't exist anymore.
When defining what private browsing is supposed to do we should keep in mind Safari was the first major browser to introduce the feature.