Maybe I misread: I thought this wasn't Register saying that Boris is "technically right", it is Boris saying that Boris is "technically right". And he isn't.
The sticker price of British membership of the EU is/was, let's say, £350 million a week. At the same time, the EU spends money on benefits to British activities such as agriculture, so the money comes back to Britain.
That isn't the "rebate". The rebate is that we got a perpetual discount on the membership price, theoretically "thanks" to Mrs Thatcher but let's face it she probably spent the money on wars and abolishing British industry.
So there isn't a £350 million cheque being written to the EU in the first place. It's £275 million, which is still handy money but a lot less.
You can see why a careless or stupid person would think that the EU costs Britain £350 million that's the price tag. But it's not what we pay. Why a more intelligent and better informed person would keep saying that it's £350 million, is, clearly, that they expect to benefit by lying. For instance, by keeping up the lie, some people may believe, or may choose to pretend to believe, that it's the truth after all. Although it isn't. And then there's the "appearing to be careless and stupid" thing, that you acknowledge if you admit the mistake
I say Boris Johnson isn't as stupid as he seems, and that's rather frightening.