The part "simply because admins of the Yahoo Developers Network were particularly slack in keeping their blog software up to date" does not adequately describe the scope of the failure at Yahoo.
Separately, it is worth noting Gmail has had a security feature for a long time which can alert users to possible sessions left open accidentally on other machines. The same feature provides a logout all sessions button which you can hit anytime for even a slight doubt.
It appears that Yahoo does not provide any information to the user about the possible existence of other sessions and requires a password change to effect a logout of all sessions.
Edit: The password change requirement itself is not a bad thing for most users and is a sensible implementation. If it is not a session steal then a user may have chosen the 'save password' option if they were physically present at the other machine. Just saying Yahoo lacks the additional control and insight that Gmail provides in this area.