Life is not a game - games are life.
I believe it was the Shut Up and Sit Down playthrough video (of Catacombs 3E, not Catscombs and Castles) in which they mention this rule.
SUSD mention this rule in their Flick 'Em Up review (which also features Catacombs 3E) because it is specifically addressed in the Flick 'Em Up rules. This is not a specific rule in Catacombs, however.
In practice, I've never found flicking by using your thumb in this way to provide any advantage or to even be particularly useful; Catacombs is a game of finesse - not brute force.
That said - to each their own. Play it whichever way makes it the most fun for the players in your group.