I don't actually know if intent is sufficient in this case. So maybe if nobody actually fell for it (or they did, but the government didn't bother to submit any evidence of this) then that would excuse it in law. 4900 people might have texted the number, but maybe half of them didn't have a vote, and the other half figured it out later and ultimately did vote. Maybe not, but if you have a sworn statement from someone that they didn't vote because of this, then there's no ambiguity about whether the harm actually occurred.

“It’s just a joke bro” the ultimate criminal defence.

The prosecution established the defendant’s intent to reduce the turnout of black voters “ For example, on November 1, 2016, in or around the same time that Mackey was sending tweets suggesting the importance of limiting “black turnout,””

That’s mens rea.

Something being a meme does not make it a crime, but a crime being a meme does not make it legal.