The thing is, tickling can be flirting, and it is often the ideal form of it: when people transition from being "just friends" to being intimate, tickling expedites the change, as it is often percieved as being merely mischevious. But think about it--no other ostensibly "harmless" social interaction allows one to touch another with whom he/she is not necessarily intimate quite as much as does tickling, and have it be percieved as innocuous fun at that.
For me, tickling is always sexual. Always. I don't tickle anyone about whom I wouldn't be comfortable fantasizing later on because I can't divorce the sexual and the social components. So, if I tickle someone, it's usually because I find them attractive--which means I don't tickle family members and a good amount of friends.
That being said, I still agree that it's wrong to say that tickling is always sexual. Many of my friends tickle one another all the time, guys and girls included, many of whom I know for a fact don't have a tickling fetish (yes. for a fact). When they tickle one another, it's just for fun.
Tickling can be a lot of things. Sexual, friendly, mean-spirited. . . for most people, it's many. but for the rest, like me, and, i suppose, the individual who posted the youtube video, it's one.