Hi Oldbmwcoupes,
With respect, this car wasn't really a reflection on values of M1s, although the car looked good on paper, it had lots of issues.
For starters, all the federalization was still present, inside filler caps are really undesirable, the changes inside for federalization were unique to say the least, certainly not what the normal people who did this work did. The front bumper was a beautiful work of art, must have cost a fortune to make a three piece metal bumper that looks correct with shocks behind, but of course wrong.
More importantly, somebody had done something very odd with the engine number, almost looked like somebody had tried to weld in that area, very odd, would have been better with no number or the wrong number, but what looked like trying to hide something, scared people.
There were also condition issues.
Good M1s are very strong at the moment and are holding at the prices you mention above in the past, I know of three great no excuses cars that have sold privately above $1M in the last couple of years. Sadly, most M1s you see in auction have issues, even with its issues, many considered this sale above value in todays market
Apologies for being a geek on the subject