Green, unripe coconuts do contain a watery fluid, often called milk, which is one of the most refreshing drinks I know - fresh from the tree, the husk ripped off and holes punched in the eyes to let the drink out. However, this fluid certainly does not thicken into "milk" as the nut ripens. Instead, it slowly dries out, so that the ripe nut is quite dry inside (like any of the ripe coconuts sold in Britain).
Coconut "milk" is not a natural liquid, but is made by steeping the grated flesh of the ripe nut in water, and squeezing it to produce the liquid used as the base of most south-east Asian curries. Toddy is not a strong drink made by fermenting the non-existent coconut milk (I think Mr Botsford must have been thinking of Marlon Brando's seduction of Jean Simmons with dolce de leche in Guys and Dolls) but from the sap of the coconut palm, which is allowed to ferment naturally for 24 hours to produce a weak and watery sort of beer.
In Malaya in the Fifties and Sixties, on the rubber estates, all the adult South Indian male estate workers were allowed a ration of one pint of this toddy each day, the toddy being supplied by a contractor. Any left over at the end of the day had to be destroyed. I was never sure whether this was because it became dangerously strong after longer fermentation, or because it went off and became poisonous.
John R. Gosden