'Kin 'eck! This boat is a proper 'Cold shower/£50 notes'.
Been down the V8 frost f'ked road myself, it's not uncommon or that simple to avoid.
As mentioned, you may've had a hairline crack in the block due to frost, which has held out for most of the summer, but has finally let go. Emulsified oil somewhere would be the giveaway. Mine blew mousse out of the airfilter.
For future reference, getting all of the water (fresh or sea) out of a vee engine is close to impossible. I used to remove the thermostat and run antifreeze through until it blew blue out of the exhaust and then switch off. The added bonus being, antifreeze has corrosion inhibitors in. The job is a total faff, but at least you can relax and not study the temp forecasts all winter.
Have the repairers explained exactly what the frost incurred damage is?