True, but it doesn't mean that he "challenged the status quo", unless by that you mean he wrote the same stuff as everybody else but in an easier-to-read format. The credit Zinn gets for being some sort of maverick attacking a standard view of history is... well, he wasn't. History's complex and often not very nice, as most historians will be happy to tell you, and American history is no exception.
From a purely historical context, you're right of course. I think the "challenged the status quo" part comes from the way he combined the popularization of a history that had previously under-represented in the mainstream consciousness with his activism in a variety of forums.