Jorgenson, et al, consider the exemption for income on the flat income tax, but do not consider the exemption for consumption in the "Fair Tax". Jorgenson assumed a ~15% rate with no rebate.
Which tells me you didn't actually read the paper, as Jorgenson does not assume any rate for flat tax or the NRST he modeled.
Revenue neutral tax rate is an ouput of his IGEM, not a input and those rates vary in time with variation in economic conditions that are output of his model to maintain equivalency between the baseline '96 income tax system and the tax system under study.
Again, I think you're misinterpreting the paper. (Actually, this one sounds a little more like poor reading comprehension.)
Fom Jorgenson, et al, page 25:
2. Figure 4 compares the consumption tax rates for revenue neutral substituion of the Armey-Shelby Flat Tax (FT) and the National Retail Sales Tax (ST) for existing income taxes. The Flat Tax rate is 25.1 percent in the year 1996 and remains virtually constant through the year 2020. The National Retail Sales Tax rate rises from only 15.7 percent in 1996 to 21.4 percent in the year 2020. Only the Flat Tax includes a system of personal exemptions, so that the tax rate is considerable higher, especially at the initiation of the tax reform.
As I wrote, Jorgenson, et al, assume a ~15% sales tax rate without rebates. The only reason the Flat Tax rate is higher is because Jorgenson, et al, included a system of personal exemptions for the Flat Tax. Jorgenson, et al, did not include a system of personal exemptions for the Sales Tax (rebates). If Jorgenson, et al, had included an equal exemption for income/consumption for the FT and ST, then the ST rate would be the same as the FT rate.
If Jorgenson, et al, had considered equal exemptions for the sales tax and the flat tax, their comment on page 24 becomes operative again: "This greatly simplifies the tax economist's task, since the economic impact would be the same for [both the sales tax and flat tax] approaches."
Again, I reiterate my point. The sales tax and the flat tax are "economically equivalent". They impact foreign trade in the same exact manner. Both taxes are border neutral.