Yes, it's true. And the reason is because eggs need high humidity during lockdown and especially once they actually start hatching in order for the chick to be able to turn inside the egg and zip its way out. When you open the bator your carefully built up humidity will drop, and once you close the bator again it will take some time for the humidity to build back up. This drop in humidity can cause the membrane of the eggs to dry up and the chicks, ducklings, whatever, to become stuck inside the egg and unable to hatch on their own.
If you don't know much about incubating, it's a really good idea to search this forum and read for yourself all the various sticky posts and incubating guides that people have spent loads of time writing especially to help out other people like yourself who want to learn to do it right.