What he's saying is you're not loop-sampling an "original" song, you're loop-sampling someone else's flip of a song -- which is still done and a viable option, but it gets you more and more into a grey area because now your beat is a loop-sample of a sample flip of an original song.
Just something to be aware of because, even though a lot of us flip and use samples illegally, I could understand animosity from other producers if you take someone else's flip and just switch in your own drums/bassline/etc but keep their body.
Like I said, it' becomes quite a grey area.