This is no doubt true.
Under Ballmer, Nadella "led a transformation of the company's business and technology culture from client services to cloud infrastructure and services." [0]
But the number of failed mobile (phone, small PC) initiatives and products, from long before the iPhone to multiple waves of multi-billion dollar write downs afterward, (phones, music players, ...) despite Ballmer clearly wanting Windows "everywhere", and no limit on his spending, was just as large of an opportunity, cyclically bungled for a couple decades.
I had one of the earlier generations/extinct-species of Windows phone. At the time I had given up on Palm following through on their great start, but found the Windows phone was just frustrating in other ways.
(I did have friends with Microsoft's last phone, and they really liked it. Just too late.)
Apples market cap today, approximately its iOS market cap, is a good proxy for the ball Ballmer couldn't stop dropping. Even when only his team was on the court.
So I give Ballmer a 5/10. :)
But any massive hypergrowth business, is still a massive hypergrowth business. Microsoft gets 10/10.