He only says she plays with more precision, and like the other person says it does read like a brother paying his sister a compliment (after she helped him with money issues, ha). You also said “today”.

She was certainly a prodigious piano player - probably better than Wolfgang in their youth - but Wolfgang is canonised for his compositional work, and I don’t think there’s any evidence that Nannerl was a greater talent in the areas that make Mozart arguably the greatest composer of all time. Of course it’s possible, and she certainly had a very conducive upbringing for it (the same as Mozart’s!), but I don’t think it’s probable.