Well, the VIX futures trade well after 4:15. What I don't understand is that the collapse didn't happen just in the last 15min after the session but progressively up to a an hour or two after if I remember. The underlying S&P VIX strategy seems to be using a fixing as of 4:15 indeed per the S&P documentation [1].
In any case, setting the fixing in the middle of an illiquid market was a moronic decision and if I had been burned by this, that would be my potential avenue for litigation.
Also I wonder how much of this short squeeze was amplified by other market participants who anticipated this event could happen. Some trading desks made a killing that day.
[1] https://us.spindices.com/indices/strategy/sp-500-vix-short-t...