I'd seen the same statistic pointed out, that the rise in arm's share price post-Brexit vote exceeded the 'devaluation' of sterling, ie the vote did not make arm a bargain.
SoftBank seem to be insisting that the vote was not relevant in their decision to buy arm, and I have seen no evidence to the contrary. IMHO Arm have been an attractive buyout target for ages, it was only a matter of time. I too liked them being British, but we can't always get what we like.
I've been following these issues for a while as I should confess I'm biased, being a small shareholder myself. Emphasise 'small'... I won't be buying a yacht, nor a second home, not even an extra holiday. But the 'windfall' might take a little of the pain out of some outside painting that's overdue, and that I can't do myself owing to fear of ladders.