If it counted UTF-16 code units that would be dumb. It doesn't. The cutoff was deliberately set to keep the 140 character limit for CJK but increase it to 280 for the rest. And they did that based on observational data.

https://cdn.cms-twdigitalassets.com/content/dam/blog-twitter...

https://blog.twitter.com/en_us/topics/product/2017/Giving-yo...