there are many scripts to detect private mode on browsers (with varying degrees of success),
just checked and whatever MIT use either doesn't work or deliberately isn't implemented for the UK (maybe it might conflict with GDPR).
Of more concern is *why* MIT does that in the first place without any transparency, even many other USA sites explain (or whine at you) that privacy tools reduce their ad revenue (techdirt read from Europe is an example)
This is very simple.
A simple reason for this could be that at some point the website uses the browser's local storage, which does not work in private mode.
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