Well, that was still my first thought when I hit "without other sources of iron". One bottle per day of Soylent can't fix up the variation in the other 75% of your calories. I don't call Wheaties unhealthy for being low on protein, because balanced nutrition has to be calculated across a whole diet.
Having said that, it is alarming that Soylent can't possibly be a balanced replacement for everyone. A custom product would have been an interesting possibility for doing that.
The everything for everyone thing is the cardinal sin with them for me.
If they marketed it as a convenient meal replacement, like a carnation instant breakfast that wasn't a milkshake, it would be different. But they imply that eating food is bad, just drink Soylent.
Having said that, it is alarming that Soylent can't possibly be a balanced replacement for everyone. A custom product would have been an interesting possibility for doing that.