This was a real math bug after all, not just a typo.
The fibre plant amount calculation was failing, but the text message referred to correct amount - which can vary depending on the plant.
It's fixed now, and the mininum amount of plants to get "practical amount" ie. one bundle of fibre goes as follows:
5 flaxes
8 hemps
15 nettles
Fixed - persists in 3.72beta, and likely in earlier versions too..