It's clearly used as an adjective here, which is the pejorative use of the term. It's even packaged in quotes, which alludes to the colloquial form of the word.

It's clearly used as an adjective here, which is the pejorative use of the term. It's even packaged in quotes, which alludes to the colloquial form of the word. If you were to use it properly, you'd say "societal progress has been retarded by the U.S." you wouldn't say "the U.S. has done something retarded"