True, but it doesn't change the fact that UK headquarters wouldn't legally be able to use EU profits without them having been taxed in the EU before. Apple needs its European headquarters to be within the EU. They might use London for profits made elsewhere (non-US, non-EU) that now run through Ireland.
It depends on the terms of Brexit - which is yet to be started and negotiated. If UK will end up like Norway, it will be very similar to being in the EU in terms of taxes and customs.