From that link:

Again, to reiterate, this type of chop block is legal. On a running play, an offensive lineman can engage a player below the waist provided he is not more than one position away from the initial block. In layman’s terms, the right guard can chop the nose tackle engaged with the center. The right tackle cannot.
So the Colts-Texans call was because it was a pass. The Seahawks non-call was because even if you judged it to be a chop block (two guys engaged on one defender, one high and one low), its legal if its a run play and they are adjacent.