You're all half right.
Cuban legitimately took a fledgling Audionet / Broadcast.com in 1998, grew it meteorically, made some good content deals, and 18 months later sold it to Yahoo for $5.7B in Yahoo stock, of which Cuban's share was ~$750M. But he did one brilliant thing right then, which was to make
a huge collar option trade on Yahoo's stock that basically locked him into its present value at the time of close, despite him being restricted from selling for a period of time. So he preserved his wealth through the dotcom collapse. He's not the savviest tech entrepreneur I've ever seen (nevermind the smartest or most charismatic), but he's a legitimately good businessman who built a couple businesses and made one brilliant move with his own money.
The best move Ballmer ever made was deciding to work for Bill Gates in 1980 as Microsoft's first business hire (employee #30), dropping out of Stanford GSB. He was a successful manager there for 20 years across a range of divisions, mostly overseeing sales. He was in charge of divisions but had nothing to do with design or tech decisions. As CEO, Ballmer did a good job defending the Windows and Office turf, but didn't undertake the bold ventures that he should have, particularly around smartphones, and a lot of his acquisitions flopped. I'd call him a very good manager but not a great leader.
I would not want Ballmer leading my VC fund, but if he wanted to turn around an underperforming business or division I'd be all for it.
Both of them would have been rich even if they'd been unlucky, but a little luck made them billionaires on top of it: Cuban with his Yahoo deal, Ballmer by latching onto Gates.