They weren't possible. A quick Google says that median US home price in 1980 was $64k, and the average mortgage interest was 13.7%[1]. Plugging that into a loan calculator, assuming a 20% down payment and 30 year term, and I get a $600/month payment, almost the entirety of that worker's full time $4/hr salary.
Either the numbers here are spun or Orem was extraordinarily cheap.
[1] This was right in the middle of the inflation crisis!
