When we use the words “rate limiting” or “rate limits” on this site, it means a very specific thing where the API itself limits the amount of calls to that resource, and programmatically informs the application what those limits are. In the Twitter API, at this time, only GET-based requests are rate limited.
Every Twitter account has their own allowances for write actions. These allowances are not API allowances, but intrinsic to the account themselves. When a user tweets from Twitter the website, or an application, or through a widget – that user’s allowance is decremented.
So, the method statuses/update itself is not rate limited – but the user’s ability to send tweets is.