The up and up brand of disposable diapers are not biodegradable which is also true for many other disposable diapers that are available.
No matter what a diaper company claims no disposable diaper is 100% biodegradable and will likely still take 100 years or more to fully decompose.
Still though since disposable diapers have not been around 100 years or more there's now way to know exactly how long a disposable diaper will take to decompose in a landfill.
Some disposable diapers may use more ingredients that biodegrade and decompose a bit faster than other diapers but all disposable diapers contain plastic backing underneath to keep the urine and moisture inside.
So that plastic covering will take many years and possibly centuries before it breaks down if it ever does.
So no Up and Up disposable baby diapers are not biodegradable although they are good cheap diapers and work very well at absorbing urine and keeping baby dry and comfortable.