It just means it can't be played by what you currently have installed.
I don't know enough about that particular video format or the involved codecs to know if there is a codec for Windows that will play it, but I'd be surprised if there wasn't one.
Recommended codecs can be found in the FAQ HOW TO: Enable/fix playback of various media formats in Opus (there are some formats that may need something more esoteric, but those cover most of them).
No, but it uses Microsoft's video playback APIs.
Opus can use MediaEngine, MediaSession or DirectShow. MediaEngine and MediaSession are newer and use one type of codec while DirectShow is older and uses another.
Most third-party codecs are DirectShow ones.
Opus's Video & Audio viewer plugin can also be configured to restrict which APIs it tries, which may be useful if you have a video where one API says it can it but then fails during playback, which would allow another codec to be used. (Most of the time that is not needed, though, since the codec/API should say it can't handle something and allow automatic fallback to another API.)