Why does this custard say to boil milk cool and boil again
I can't say about this particular recipe, but 'scalding' milk was a commonly used to change the milk (cooking proteins, deactivating enzymes, etc) in the days before pasteurization ... but that was normally done when the milk was to be used at a non-boiling temperature.
It's possible that this 10 minute cool down gives it sufficient time for the desired changes to the milk to happen, without the problems of boil-over and evaporation that might happen if you tried to hold the milk at a boil for an extended period.
It's also possible that this is related to another question on here in which someone noticed that re-boiling milk was less likely to foam up.  (but then couldn't re-create it, so there might be something else going on)