Precisely... the two ideas are independent.
If his investments are publicly-traded, selling his stake does nothing. The companies he's invested in won't lose his money, because he'd just be selling to another individual, so "his money" becomes "the other guy's money". If it's a private investment, where he may be contractually limited in what he can do, then the whole discussion is rather moot. He may be able to sell his way out of the investment, which would reduce the company's operating capital somewhat, but unless he's a major shareholder, the impact on the company will be minimal.
On the other hand, if he keeps his investments, he likely gets votes in how the company operates. Being Bill Gates, he probably gets a few more votes and can bend a few more ears than regular folks can. If the investments do anything, good or bad, that's where it lies... they give Mr. Gates the ability to push the fossil-fuel companies in a more environmentally-friendly direction.