I don't see that being the reason.
Amazon know how many units they have allocated, because Nintendo told them. If someone cancels their order on their website then 1 unit is freed up and Amazon would immediately know this. That 1 unit would be available to buy again on the site. Most likely it would go to the next person in the order queue who wouldn't have been getting his at launch.
The scarcity of the system around the announcement was real, not just on Amazon but at lots of places. Nintendo always knew the absolute maximum number of consoles they could make by launch so I don't see this as 'they found out they could just make more by that time'.
But hey, it works for them, people think it's scarce and so logged their preorders in early.