I suppose you can just attribute it to imprecision, but "I am not particularly certain ...how much they exist" implies that he's talking about a subset of mathematically possible universes that do objectively exist, but yet exist less than other worlds. What you're talking about, conversely, seems to be that we should create as many good worlds as possible, stretched in order to cover Eliezer's terminology. Existence is binary, even though there are more of some things that exist than there are of other things. Using "amount of existence" instead of "number of worlds" is unnecessarily confusing, at the least.

Also, I don't see any problems with infinitarian ethics anyway because I subscribe to (broad) egoism. Things outside of my experience don't exist in any meaningful sense except as cognitive tools that I use to predict my future experiences. This allows me to distinguish between my own happiness and the happiness of Babykillers, which allows me to utilize a moral system much more in line with my own motivations. It also means that I don't care about alternate versions of the universe unless I think it's likely that I'll fall into one through some sort of interdimensional portal (I don't).

Although, I'll still err on the side of helping other universes if it does no damage to me because I think Superrationality can function well in those sort of situations and I'd like to receive benefits in return, but in other scenarios I don't really care at all.