Isn’t the UK leaving the EU?

Yes, but the Brexit referendum was only 1 month before SoftBank acquired Arm Holdings. The deal was probably already in progress, and finalised before the UK had any real policies about Brexit, so EU requirements would have been reasonable if decided ahed of time. But the timing may also explain the lack of any national security focused requirement (general confusion).

That just means that their home/core market got smaller, so they should have protected ARM to stay inside E̶U̶ the UK, then.