It's not an automatic call. There were comments on SwimSwam from swimming officials who said they have seen it ignored many times. But if anyone should know the rule it's a distance swimmer. Those races are prone to huge gaps before the final swimmer finishes. In this instance I think it was something like 50 seconds.
Swimming is absolutely picky on rules. These days you have to hold your breath after every individual medley in particular. There are frequent disqualifications for turns and kicks.
And in the recent world championships the guy who would have been favored for gold at 200 butterfly was disqualified after the semifinal for failing to do a two-hand touch. It was very borderline. His left palm touched first, well underwater. Then he pulled the lefft hand off the wall just as the right palm touched. In other sports there is benefit of a doubt. In swimming the tie always goes toward disqualification. He is a Polish swimmer who goes to school at USC. Apparently he launched a tirade on social media before being persuaded to delete the posts. He was upset partially because his twin brother was also in the final. There was a chance to medal together.