How does the Platypus grow before it eats?
The video is quite old and they knew a lot less about the species then.  They probably had to make some guesses.  Any modern reference I find doesn't mention any waiting period before nursing.

It seems to me that were this the case it would be very explicitly studied by now as it would be a real oddity for a mammal.  And we love platypus oddities.

Interestingly, the platypus doesn't actually have teets to be suckled on.  Instead the milk is expressed through their skin and into grooves on the belly that the newborns can drink from.