It is also an error to assume that disallowing GET for updates adds any security. I recommend reading about cross site request forgery. CSRF is the type of attack used on Twitter today.
The article is also subtly wrong. GET requests can be protected from CSRF attacks. There just isn't ever a reason to do that if you're doing things right.
2. You're arguing about semantics. CSRF is a security issue. Being able to send updates without user's knowledge is a security hole too. Backed up by a wrong behaviour if you wish. I should never forget that HN is a Serious Business.
None of the above mentioned makes twitter guys any less lame.