I made a mould out of Lego (to make Lego-shaped gummy candies) and with a thin coating of vaseline on the outside, it was indeed water-tight (and silicone-goop-tight too).
They're astonishingly well made for a children's toy.
That said, they've been cutting quality, moving production to China, using cheaper formulation plastic. That's a bad move imnsho. The brand image is quality and people pay for that, if they ever seriously let that go it's going to be game over for them.
Of course, chances are they'll reduce their production costs but maintain the high prices.
Do you have any specific set in mind?
He already has 31052 and 31051 which I think are excellent 3-in-1 sets. Would love to see more like those. They are about AU$80-120 which is a workable birthday/xmas pricepoint.
That's really not that much (1-999 μm). Keep in mind that sub-1mm is micrometer, and 1mm is a huge distance. I'm sure that LEGO bricks have tolerances of single or low double digits of micrometers...
The moulds are permitted a tolerance of up to two micrometres, to ensure the bricks remain connected.