My guess is they only connected the large red cable and missed the smaller one at the pos terminal on the battery 😳
Good point, I'd forgotten there's 2 positive battery cables and, if it's the same as mine, the larger one is red but the smaller one is in a black convoluted sleeve with a red terminal so maybe easy to miss if it's been tucked out of the way for transit.
If they had only connected one then that would explain why jumping the other battery to the terminal post by the oil tank didn't make any difference.
