The timing of the call was what was jolting to some fans (viewers), but the call was correct.
The Seattle offensive line was grabbing, clutching, and holding all night long. They could have been flagged a dozen times or more.
I've spoken personally to retired NFL offensive linemen with over 40 years experience between them about the game and officiating. They watched the game of course, and said generally the right side of Seattle's offensive line was over-matched all night, and in desperation were grabbing and holding all night. And it just wasn't confined to the right side of the line.
They also mentioned, as an aside, that for some odd reason, players on both sides of the ball were having a hard time breathing, were getting inordinately fatigued in that stadium that night (that sometimes happens because of the extended nervousness during the run-up to the game).
Now, don't get the impression that the Steelers didn't hold too. They did. But refs let it go until it's just happening too much, especially by one guy in particular.
By the way, down on the field during a game, there are a number of times that the officials will warn linemen and others......... "hey, next time there's going to be a flag", or something of that sort.
Seattle was just over-matched all night and resorted to what the great and witty Tunch Ilkin refers to as "Habbeus Grabbus".
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p.s. These retired NFL players also laughingly gave their kudos for the professional and sly way Seattle's receivers ran their "pick plays" (a technically illegal play) all night. One of them helped give them their only TD.