I think there are multiple differences but in my mind the biggest one is that his post mostly talks about the benefits that he gained himself. There are no promises about what it would do for you.

Now obviously if you're reading critically and you're naturally a bit suspicious of his motives then of course it seems heavily implied that the benefits he feels he's gotten are the ones that you'll get too once you also use Soylent.

There are two (maybe three) ways this can go:

1. He made it all up to launch a startup

2. Everything he says is true

3. He believes everything he says is true

In truth it's not as mutually exclusive as I've presented it. Many of the benefits might be true, he might believe some BS about others, and some of it might be outright fabrications.

I think another hugely contributing factor to the notion that he's not a snake-oil salesman is that the formula seems to be freely available and not patented: http://en.wikipedia.org/wiki/Soylent_(food_substitute)

I am not sure how you would do a double blind between a supposedly all-encompassing shake-meal and one that is not. Are you going to purposely malnourish people so that you can see they're malnourished? That's ethically dubious at best.