dipstick could be dry with say a litre less of oil than what is required doesnt mean to say there was no oil, a car which holds 5litres often wont register on the dipstick until about 4 litres
True, possibly, but there is no way to quantify the margin of error once it fails to be read on the only measuring device the car has. For this reason you can only say it is as likely as not that the sump contained no oil, particularly so if you reasonably assume that it it ejected this much in 30 miles or less. Even if it had some oil and the OP made a tight turn at moderate speed it's possible the engine suffered oil starvation.
If it's under warranty I'm not sure if it matters how it got into this state. Something has gone very wrong and hopefully it'll be insured by a warranty company so the OP's garage won't be out of pocket themselves.