No, it´s a limitation of probably something missing on your not mentioned system. Apart from the manual, there is also a knowledgebase article about video and Cubase.
Sure, you can export the audio from it, and you can replace the original video´s audio with the one you scored in Cubase.
And of course it´s a matter of definition, but no, you can not really “work” with video in Cubase.
Last Nuendo version I used was N2 and IIRC apart from the better post pro audio features, “working” with video is the same there. But there are Nuendo users around here who might chime in and prove me wrong…