Real lawyer here. There’s more than enough wiggle room for it to be technically true.

“I don’t know whether Mr. Mueller supported my conclusion”

Unless Mueller’s letter specifically says “I don’t support your conclusion” or something to that effect then it is a true statement — and from my understanding, Mueller wrote that Barr “did not fully capture the context, nature, and substance” of the special counsel’s findings, which is a general critique that doesn’t inherently question the conclusion. 

That said, there could definitely be more damning language in the letter that makes it perjury, or makes the case stronger.