Sorry to hear and hope you and the bike patch up ok … my opinion (as with some others) is you had the misfortune to hit a patch of spilt diesel … can’t see it in advance and next thing you know you are off sliding down the road … diesel or ice … there ought to be some sort of safety campaign showing drivers the consequences of overfilling their tanks and / or not having a filler cap allowing it to spill on a roundabout.
Never have a problem cornering on my T-bird 16, she regularly grinds the footpegs on roundabouts though and that can cause a bit of a wobble. It had Pirelli's on it when I got it but they were American tires and not great in the wet. Switched to Avon Cobra's and she sticks to the road like glue. Back in the day when we used to bring in American Zeds and such like the first thing done was always to change the tires as the US ones were too hard a compound, especially if they came from the West coast.