Olympic track races are covered by the rules of the IAAF. You can download a PDF from the rules page. That PDF says, on page 159,
- (a) In all races run in lanes, each athlete shall keep within his
allocated lane from start to finish. This shall also apply to any
portion of a race run in lanes.
(b) In all races (or any part of races) not run in lanes, an athlete
running on a bend, on the outer half of the track as per Rule
162.10, or on any curved part of the diversion from the track for
the steeplechase water jump, shall not step or run on or inside the
kerb or line marking the applicable border (the inside of the
track, the outer half of the track, or any curved part of the
diversion from the track for the steeplechase water jump).
Except as stated in Rule 163.4, if the Referee is satisfied, on the report
of a Judge or Umpire or otherwise, that an athlete has violated this
Rule, he shall be disqualified.
There is an exception if you're in a straight segment and gained no advantage, but the article says he was on the turn. The rule clearly says "shall not step or run on or inside the kerb or line" so stepping on the line breaks the rule.