"Maybe Soylent is okay. Maybe. But there are well established, scientific methods to evaluate these claims. I know it tempting to want to believe the claims in the video, but they have done NONE of this!"
That sounds like a pretty reasonable argument to me. Pointing out that the soylent proponent's "evidence" does not stand up to scientific scrutiny does not sound like "argument from authority".
I agree that it should stand up to scientific scrutiny before being able to claim that it is safe, but the argument he was making before he deleted all his posts was that Soylent was most definitely unsafe solely because it was made by "some computer kids".
And that's a very valid point too. If you wanted me to completely give up my entire diet and consume just one product for the rest of my life -- here's what you need to do:
1) Have some (at least one) long term, large cohort, controlled studies proving the efficacy of this diet.
2) Be someone I can trust with being careful and honest with your scientific method over a long period of time.
So saying that you shouldn't trust some mixture put together by some computer programmers in their 20's is just a shortcut to the the two statements above. He (assuming gender here) was absolutely right in his assertion.
That sounds like a pretty reasonable argument to me. Pointing out that the soylent proponent's "evidence" does not stand up to scientific scrutiny does not sound like "argument from authority".