FML would have had no relevance to his joke making sense regardless of what meaning it had.
But okay.
The sentence you just provided as a retort made no sense, if 'FML' had no relevance to the understand in one's mind of whether or not the joke made rational sense, then why did you point out its meaning?
And regardless of its meaning? If you didn't understand an entire word of a sentence the chances of you understanding that sentence that is a total of 5 words long is pretty low, with the exception of a person with very good context clue abilities.