Maybe the yeast was mishandled during delivery? There's no real way to know, but could account for a massive die off, leaving you with too little, even with the starter. This is unlikely to be the cause of your trouble since you added other yeast to finish off the beer and it never dropped points. Have you calibrated your thermometers recently? I had this happen. Thought I was mashing at 151, but really was 155... Made a big difference.
I had no idea that SD Super Yeast was finicky. I have been using (abusing?) it for some time and it always seems to do the job quickly, and in several different beers. In fact, it's my go-to yeast in the house now. My basement sits at 65-68 year round. Colder on the floor.
I have had great experience with this yeast from 4-10% beers. It has been highly attenuative, and floccs out quickly.
BTW, my brew buddy has a much colder basement and had trouble with 090 recently. It was resolved by bringing it upstairs, rousting the yeast, and it finished in another day.