The boxman was correct to call a number, rather than a "no roll," and his explanation was correct, but you misunderstood what he meant.
There's a standard industry rule covering an occurrence where a die lands tilted against a chip, and calls to Bally's, Bellagio, Caesars, and the Wynn all confirmed that it's standard practice in any dice pit. The rule is that the call on a "cocked die" corresponds with the "natural fall of the dice," meaning that whatever the "higher side" is physically (not numerically) is the number called.
In other words, the die is examined quickly (to keep it objective) to determine in which direction it was traveling and how it would land if whatever obstacle is tilting it were removed, often with the boxman verbally instructing the dealer nearest the roll to "call it." The phrase "high side" that you probably heard has nothing to do with the numerical values of the sides in question, but rather the side of the die that was falling dominantly. Furthermore, Caesars clarified that in instances of a cocked die, it's never a no-roll, because it can always be determined which way it would have fallen if unimpeded.
Instances in which a no-roll is called include (but are not limited to):
The most amusing instance of a called no-roll that we've observed took place in the early '90s at a dice tournament in Atlantic City. At an absolutely stacked (with experts) final table, 1994 World Series of Poker champion and Ultimate Blackjack Tour founder, Russ Hamilton, was first to roll. When the dice were dumped from the bowl and the call "Start play" was made, Russ quickly picked up five, flung them the length of the table, and yelled "Yahtzee!" For those who were there, the crew's screams of "NO ROLL" still ring out today. (Russ went on to win the tournament.)
