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^^^ This is correct. To be a bit pedantic, it is a measurement immediately post boil as some isomers may get trapped in cold break or oxidized out of existence on the way to the fermenter.
Garetz is the only calculation that attempted to predict post fermentation results, which is why it's consistently lower than the others. It's explained in his Hops book, which is out of print, but comes up in the used book market. His hypothesis is a little outdated because it was created assuming bitterness was the product of the boil, not steep or whirlpool times.
Most breweries report that without adding isomers post fermentation, about half of IBUs can be lost in fermentation of most beer styles. Even super saturated IPA's tend to land at 70 to 80 IBUs after fermentation. A lot of this is because an IBU is a rather specific measurement, which doesn't take into account perceptive variations based on hop alpha acid type. Most of us who brew have come across a smoothly bittered 70 IBU beer and another with lingering, resinous coarseness at 45 IBU. Even an entirely unhopped beer can register 2 to 5 IBU because of grain tannins.