Well....
By the scuttle (= a hole to let in air on lower decks of a sailing ship) there was a butt (=cask) of water where the crew could get a drink - and while they were there, chat, plot and swap rumours- the modern equivalent of the office coffee machine.
It's only a jokey nickname. Both swedes and turnips are root vegetables. I picked up the expression which was being used at my many visits to the Gothenburg boat show and the floating show at Langedrag many years ago. I can assure you that it is a very friendly and affectionate nickname.
We brits aren't that polite sometimes. We have affectionate - or sometimes not - nicknames for other races. Frogs or froggies, sausage eaters, turnip heads or cloggies. What do other races and peoples call us? The french call us rostbeef - the wizards of oz call us pomms or pommie bar stewards. Who else would like a go?