Yes, but he still considers it impossible even with modern computers and algorithms.

His fundamental conclusion (see the section on non-convexity) is that it's as hard for capitalism as planning, which isn't really an issue for CEV. ('OK, so fine, we'll go with either system as convenient, and apply optimization to as large subunits as we can manage before it breaks down.')

I'm not sure how extrapolating, making consistent, simplifying, unifying, and implementing the aggregate preferences-in-general of everybody on Earth would be easier than simply implementing the resource-related preferences of everybody in a single nation.

I thought it was obvious. The difficulty is related to the number of arbitrary distinct constraints being enforced. Reduce the number of constraints, and you reduce the difficulty.

Whether CEV is actually possible - whether the reduction in constraints happens and a Parfitian convergence of ethical criteria happens - is the fundamental question and doubted by many, but also unaffected by what kind of complexity linear optimization may be!