They found out that Japanese Twitter users don’t run up against the character limit, and are less frustrated as a result. The point is to make everybody else less frustrated, not to make the Japanese as frustrated as everybody else.
Yeah, you can type a lot more in Japanese. Most characters stand for at least two US characters, which is most likely how they came up with the 2x decision.
Sadly that is not going to be sufficient. Japanese uses very few context indicators and thus as a whole is a lot shorter.
Found the UX designer in this sea of clueless engineers