Re: If they'd done that, Intel could've just paid ATi more to do it for them instead. The purchase eliminated that option.
You get around that problem by constructing the JV legal agreement such that AMD gets full exclusivity to the technology developed by the JV. By the time Intel finds out about AMD doing a Fusion processor, the JV is already signed and in effect.
At any rate, the point is clear. Any joint venture done by AMD and ATI would have cost AMD far less than the $5.3B purchase price, plus $1B in restructuring charges necessary to go through with the buyout.