From
dictionary.com: Imperialism: The policy of extending a nation's authority by territorial acquisition or by the establishment of economic and political hegemony over other nations.
The system, policies, or practices of such a government.
From the Oxford English Dictionary: "anti-: ...opposite, against, in exchange, instead, rivalling..."
For a man who openly stated in his letters that he disliked empires; and who portrayed the Númenoreans' gradual fall partly by having them begin to adopt an imperialist philosophy, 'anti-imperialist' seems like a fair description. Of course, it's by no means complete, but I think that it does sum up his views on empires.
The author of that introduction would not be the first to hijack the ideas of a novelist and use them to promote a personal agenda. Yes, his views are extreme, but they're not incompatible with Tolkien's. Personally I think that he gives imperialism too much space in that foreword, and that he could have been a little less confrontational; but clearly he wanted to make a point: that we honour people for actions without really thinking what those acts entailed. He is not wrong to do this, but in my opinion he ought to do so somewhere else. Tolkien was against imperialism and cultural domination, but not so much so that it is absolutely at the centre of his work. It should have been enough to note that Tolkien disapproved of the British Empire, draw in a point about the changing attitude of the Númenoreans to the Hither Lands and move on to another theme. Anything else says more about the writer of the foreword than it does about the author of the book.