Between carbonization and electroplating, a substantial short circuit could be formed. Remember this is DC, so ionic transport is a thing, and ions can be plated off one electrode onto the other, usually in a haphazard way (in a foamy deposit due to excess hydrogen production, or in dendritic crystals without an agent to encourage smooth deposits). There shouldn't be much electrolyte available, at least not initially, but that just means it'll take longer and have higher resistance; and cell voltage is in no short supply here.
The coolant leak could've been among the battery itself, which logically should occupy the bulk of internal volume regardless of where the controls/inverter are placed, and obviously has to have lots of metallic connections, not necessarily exposed connections, but would they take the time to seal everything up when it's already supposed to be nicely contained in pipes, and the enclosure(s) already handle rainwater?
Tim