Well, one song is hardly defining of a bands entire output. It's not exactly a song - or even an album - that I'd classify as the "core" opeth sound. Like, Ozzy has a handful songs that are blatant Beatles worship. Like "Goodbye to Romance", but I'd hardly dismiss his discography as merely ripping off the Beatles.
Something like this is what I'd call full-on Opeth:
https://www.youtube.com/watch?v=rggQNCS8UjM
Obviously old school prog inspired, but that's one thing... and being a blatant ripoff with no expression of one's own is another.
In any case, I'd have to listen to Mirage to see if I agree.