No disposable diaper is 100% biodegradable because all disposable diapers do use some plastics to help keep the urine in to make them leak proof.
Still some biodegradable disposable diapers use wood pulp for the absorbent portion of the disposable diaper instead of the SAP "Super Absorbent Polymer" which makes the diapers a bit more biodegradable.
But still even biodegradable disposable diapers will take a very long time to break down in a landfill.
I still would use disposable diapers instead of cloth diapers because cloth diapers require too much work and are not as comfortable on the baby as disposable diapers are.