That was kind of the same question I had when I read it, but I came to the (completely uninformed) conclusion that probably, the body's absorption rate was self-metering enough that it probably wouldn't make a difference drinking it all at once vs. drinking it over a 20 minute period.
I have absolutely no evidence whatsoever to support this however, so who knows?
Fructose is metabolised by the liver, which may only be able to handle so much at a time without negative effects. Raw fruit is digested more slowly than juice due its fibre content, so drinking juice slowly could be worse for your teeth but better for your liver.
Even modern raw fruit are significantly larger and sweeter than a few hundred years ago, so maybe we should just stick to berries.
