I would still offer grit to be on the safe side. They will only eat it if needed. One hour may not be a lot of time to pick up enough grit. Mine lay around a lot when they are outside.
As well, my ground was frozen up until two weeks ago. Harder to get grit from the frozen ground.
As per the link you posted:
Grit: Ducks that have access to dirt will pick up small stones on their own and don’t need supplemental grit offered. If your ducks are kept in enclosures without access to dirt, sprinkle a small amount of grit on their food once a week.
You would offer them chicken grit. It is like tiny pebbles. It helps them to digest their food like it helps chickens. I am not talking about oyster shell that is not grit. Oyster shell would be offered for calcium.
I will go read the link you posted, but I read Storey's guide to raising ducks...
@QuakieChan your duckling probably is lonely. But also, it would eat the treats over the crumbles. It is like being offered candy and then liver. :p If you are offering too many treats, you might want to cut back on them. He will eat when hungry.
As long as everything else is ok with it, it...