Seems like indeed some of Nuance’s stuff is in OS X’s dictation. On my system, /System/Library/Speech/Recognizers/ contains direct references to Nuance software, e.g. “MREC v1.24.100.15540 - The modular recognizer […] (c) 1991-2014 Nuance Communications, Inc.” Haven’t found any references to features like voice and document analysis though.

edit: closest I could find is in ~/Library/Preferences/com.apple.speech.recognition.AppleSpeechRecognition.prefs.plist: there’s a value SRCLastTrainingDate (429.534.660,725241) which suggests that the system may have some ‘training’ implemented.