[lxgr]

Yeah, I guess I don't see what's funny about that statement.

Unlike "sometimes a second is longer than a second" (not literally true but it makes some sense in the context of relativity), this one just seems like a tautology to me.

[kijin]

Yeah, it might not be funny, but the tautology draws attention to the fact that there is no privileged frame of reference.

In other words, the only thing we can say without qualification is that a second is just a second in the same frame of reference. All other statements must be heavily qualified.

Even things like "A's second is longer than B's" are only valid in some frames of reference and not others.