I think the lifting background is biasing you. I'm also a lifter and I had the same thought when I first saw Soylent. But actually, I would dispute that fat is cheap and plenty. Carbs are for sure, but when I do the math I'm actually very unsure how a typical American diet would have either enough fat or enough protein, without scaling the whole thing to well beyond caloric maintenance, which is probably exactly what people are doing. Unless you're a serious lifter, the macro ratios in Soylent actually make a lot of sense if you are eating it for most meals.

You're right to point out that there is a bit of a conflict between Soylent's two goals of being both a viable replacement for every meal, and for any given meal. If you only eat one meal of it, it should probably counteract the bias of your other meals, namely, having too much carbs. But keto-Soylent would not be ideal for everyone using it as a complete food replacement.