They just said it was too hard for their feet to stand on. That wild ducks don't spend nearly the same time on their feet, and when they do they are on grass or dirt.
This is 100% correct. By domesticating ducks, we've also significantly increased their weight, therefore more weight is applied to their feet, so it becomes even more crucial to provide them with a soft bedding material to the best of our ability, as wild ducks are lighter in the first place, and for the most part are either in water, or on uncompacted soil.
A lot of the times Bumble foot is just unavoidable, but taking some time out of the day to consider what might be the underlying cause of the bumblefoot is likely just as important as treating.
Usually that tends to be due to hard compacted grounds, but can also be avoided by correcting any nutritional issues, providing dry/clean bedding, or taking care of any preexisting conditions the duck/s might have that is enabling them to put more weight one foot more then the other, or to be inactive in general.
AS far as the pictures you provided, Farrah seems to be the one who actually has Bumble, the rest have some sores that could develop into Bumble (so they are prone), but it might be able to be prevented just by correcting any management issues, since you caught it very early.
There are a few different ways of treating Bumble, so choose whatever technique you feel comfortable with. What I might suggest for Farrah, is scrubbing her foot off with a soap water or an antiseptic like chlorhexidine, or diluted betadine. Then applying a generous coat of a triple antibiotic ointment to the area, bandaging the foot, taking it off a few days later, and assessing if the core in question can be removed easily with the use of a pair of tweezers and scalpel. Of course, after the core is removed, more badaging will be needed as a protective layer as the wound will then again be prone to reinfection.
https://inpractice.bmj.com/content/36/7/356


