That's not ice scour, that's megaflooding (as on Mars). A single glaciation would have erased this. There was also 2 km of ice here, this is just the bottom fraction of the crater. At least you admit it's an impact remnent, so that's a good start.

As far as the existance of the great lakes refuting a 12,900 BP cometary impact, which is now well supported by the greenland ice core evidence, I must admit I really have no idea what you are talking about. I will admit that this particular astrobleme could have happened at any time during the last 100,000 years, but since I was looking for an astrobleme that produced megaflooding on this side of Lake Agazzis WITHOUT causing a massive drawdown of the lake, then this fits.