This is not April Fools!
In all honesty I'm excited about this choice because Johnny Knoxville is a funny actor and crazy-stuntman! I've always been a fan of his, perhaps I'm being biased but I think the guy is great!
Now this could be a rumor but I highly doubt it because the actors in the suits were only there to be "physical actors".
According to Deadline:
If the half-shell hero Leonardo sounds a bit like Bad Grandpa in the upcoming [Teenage Mutant Ninja Turtles](movie:323027), and the half-pint sensei Splinter sounds like Monk, there’s good reason. Johnny Knoxville has just been added to voice the role of Leonardo, and Tony Shalhoub is voicing Splinter, the Yoda-like creature that teaches chop socky technique to the turtles. Now, Pete Ploszek played the role of Leonardo during the filming part, while Seinfeld‘s Danny Woodburn played Splinter. Insiders tell me they were always open to changing the voicing component of the Jonathan Liebesman-directed film that Paramount releases August 8, and those actors were only guaranteed to physically perform the roles in costume. But as it has proven with World War Z, G.I. Joe: Retaliation and Noah, Paramount also is a studio that isn’t reticent to adding ingredients to the souffle after it has risen, and right up to the point where it’s ready to be served