[thaumasiotes]

> If you specify who's acknowledging it then the joke doesn't work.

Not quite, the original is very explicit about who acknowledges the truth in question.

What do you think is the difference between "it is a truth universally acknowledged" and "it is a truth acknowledged by all"?

[mrob]

If you find a single counterexample then "acknowledged by all" fails. "Universally acknowledged" is more vague, so you can claim you meant universally acknowledged only within a certain group of people. The source of the vagueness is the use of the passive form. The sentence is funny because the narrator is exploiting this vagueness in a self-serving way.