Because the cache key for the site is partitioned by top-level origin in modern browsers, they wouldn't get any additional information this way that they couldn't get with existing first-party storage techniques, such as service worker caches, session cookies, IndexedDB, etc. See e.g. https://developer.mozilla.org/en-US/docs/Web/Privacy/State_P... for example. Opening a new incognito window would trivially defeat this method of "tracking". This is basically just a very small first-party-only cookie.