In my opinion that clip was one of the very few ever marketed as non-con that was actually non-con.
It was also probably the single most disturbing clip in the entire history of tickling videos as well, but that's a different subject. Yes, we've all endured far too many non-con morality debates, so I appreciate that this question is instead in the "what really happened?" vein.
Why did I think it was real? On a technical level (I'm a former television producer, and documentary filmmaker) I vividly remember not just how incredibly gradually the adverse reaction slowly built over 20 minutes. in tone, language and volume, which would have been hard to seamlessly stage with an "actor." But I also remember how many times the early conversation was delivered in an "off mic" style -- spoken at lower volumes without regard for whether it would be sufficiently recorded on video. Generally people who know they're being paid to be videotaped in a staged scenario will speak in a clearer, louder and more direct way, since they're well aware that what's captured on-camera is the whole point of the situation.
That said, I'm kind of amazed that after all these years, we in this community (as far as I know) still don't know the true story of what really happened in that video, and the TickleSlaves series as well. Whether the actual true stories of those videos were fun, ugly or anywhere in-between, I'm kind of amazed that we don't know them. I'd be fascinated to hear the real stories, in case the actual producers ever read this -- DM me.