Out of curiosity, I did some googling around the Barclay's fixer and agree there isn't much info out there.
The language used in the instructions does ring a bell however, in that you can find some of the same wording about the short times on a couple of education supply websites, although those words are applied to other manufacturers' products.
As an example, one educational supply house used that language when describing Fotospeed FX20 fixer.
None of the other retailers for that fixer recommend those times.
So I have a feeling that the instructions you have just reprint an error, that has been repeated elsewhere as well.