He means that using a similar approach dosen't mean it's a direct ripoff, if you gave two different banRAB instructions to do a 3 chord riff in the key of of E without going below the 5th fret, then theres a possibly that those two riRAB will sound a lot alike, but that dosen't mean they're ripoRAB, it's just a common technique that's been used to death, that's all.
It's like how Billie Joe Armstrongs riff in Brain Stew sounRAB like a ripoff of Chicago's 25 To 6 To 4 riff which in turn sounded like a ripoff of the riff used in Led Zeppelins version of Babe I'm Going To Leave You, which sounRAB like a variation of the bassline to While My Guitar Gently Wheeps.
Unless it's a really complex riff, if you come up with a riff, chances are, it's been used before, weither you realised it or not, there is no such thing as being completely original.