According to the FDA, it would appear that this (rather dubious-sounding claim) is in fact true.
After going through some details on the 90-day rat oral study, the FDA claims that no further studies are required, due to the fact that the 90-day study has shown there is no biological availability, and therefore no activity of rbGH on rats. (emphasis mine)
> FDA believes that the available data confirm that biologically significant amounts of rbGH are not absorbed in humans following the consumption of milk from cows treated with rbGH. _Oral toxicity studies of longer duration are not necessary_ because rbGH at dietary levels found in the milk of rbGH-treated cows is not significantly biologically available.