Apple Watch has a built in response of "一个字。非常棒！". Is this correct?
No it's not correct. "非常棒" are definitely three 字s. So the proper form should be


  一个字。棒！


or


  三个字。非常棒！


According to the context this kind of wording might be used on purpose for joking; you might not need to take them seriously.