How is Hotmail exploiting a vulnerability in your non-Windows PC?
It isn't. The vulnerability to which I refer is in Microsoft software that runs on a device other than my PC.
Hotmail goes through Microsoft's server. If the Microsoft software running on Microsoft's server has a vulnerability, the data stored on said server on behalf of its users is affected, even data stored on behalf of users who do not run an Windows operating system. Therefore, shunning Windows will not protect users from all vulnerabilities in Microsoft software.
Unlike Google with respect to Gmail, Microsoft promises not to use the text of emails stored on a Hotmail user's behalf as part of Microsoft's interest profile on a user.