I would define a Silicon Valley company as a venture funded startup that leverages tech and has a set of ideologies that is usually left leaning American, usually values growth over profit, usually is fine with taking the law less serious if it's in the way of growth or profit, usually has elements of hacker culture, ...
... or any company that started out this way, which is why Microsoft can be considered to be a SV company.
Except that Microsoft didn't take venture money until right before IPO, has always valued profits over growth, and has sort of the antithesis of hacker culture. (See eg. Bill Gates' famous "Open letter to hobbyists".)
... or any company that started out this way, which is why Microsoft can be considered to be a SV company.