It is not true that every hand with a seven-card suit but few high-card points is suitable for a preempt. Hands with 7-3-2-1 distribution are typical; some 7-4-1-1 hands may be too strong; hands with 7-2-2-2 pattern are often the worst.
Players often preempt despite a flawed hand, perhaps reasoning that they have two opponents to fool but only one partner. Many players would have opened four spades as South did, but he had side honors and poor distribution. I'd have passed, intending to act later.
When West led the king of diamonds. South took dummy's ace and counted four quick losers; hence he couldn't start the trumps: He cashed three hearts to discard his diamond loser. South next ruffed a diamond and led the king of trumps.
East won and defended carefully: He led a low club to West's king, won the club return and continued with his last heart, promoting West's ten of trumps. Down one.
"I could make it," South said. "After I take the top hearts, I can avoid the trump promotion by leading dummy's last heart myself and throwing a club."
"Then I also throw a club," West remarked. "East leads the ace and another club. You ruff, but when East takes the ace of trumps, he leads a third club, and my ten of trumps scores."