He acts on what he perceives as mispricings in the market. That these mispricings may be caused or magnified by his own actions in the companies he is involved with isn't really the point.
Yes but most of what Buffett does is not activist shareholder action or buyouts, it's security/company analysis. Then once he finds an underpriced company (as was the most recent case by Heinz) he then takes an action to get the best price for the shares for Berkshire. The market won't digest the information of these private negotiations of course but up until that point Buffett's idea generation is largely driven by the public price of securities, at least according to what he tells us.
The efficient market hypothesis does not preclude the possibility that share prices could be improved by a buyout or activist shareholder.