In bridge, how low an "unlucky" percentage would a good player guard against?
2%?  I don't think most would worry about 2% on most hands, because most hands you'd be losing a higher percentage of the time to protect against the lower.

But yes, in this kind of case it's not uncommon to protect against 5-0 splits.  While it's not common, 2% is 1 in 50 hands, meaning it might come up a few times in a weekend; the difference between a highly experienced player and a top-tier player might only be a few hands, and those would be some of them.

It really comes down to the hand.  If the hand is played at IMPs (where making the hand is paramount, rather than overtricks), and it's solid for 4-1 or 3-2, then there's no reason not to explore 5-0 split lines.  I've certainly done it a number of times before, and I'm hardly a world class player.

I suspect your story is a bit of an apocryphal one, given the description of the actions after the tournament director was called are inaccurate to what would really happen at a tournament (slow play involves slow play warnings, and eventually board penalties, but would not modify the results of a hand; the tournament director would not "award the point" either, it would be up to the opponents to accept the claim or not, if a claim was indeed what was made).  But the actual details - that a world class player considered his line of play for several minutes in order to find a safe line against a 5-0 split - are entirely reasonable, if a bit boring compared to the dramatic telling.