It's not true. It's actually even higher at close to 100%.Is that true? Has Ace Attorney and Judgement lied to me!
IIRC those games don't really paint a different picture, though. Both of them treat a not guilty verdict as something that's absurdly rare. "Has never lost a case before facing Phoenix Wright" is essentially the default background for any AA prosecutor, and Yagami getting his client declated innocent at the start of Judgment is basically considered a miracle.