... as opportunistic [behavior] Adjective - opportunistic Adverb - opportunistically Noun - opportunist

It's an adjective. It's modifying the unmentioned or suggested "timing" or "moment" that he chose to do the deployment. If you reword the sentence it might be clearer: - Some observers thought that he chose an opportunistic moment to deploy it; his last day in the office. - He chose an opportunistic time to do it.

It's usually used as an adjective, but could be used as an adverb (used to describe an action).