I've never heard of anyone keeping them in a tank without a cover before, simply because it's better to be safe than sorry, however on my research journey I did find this interesting tidbit:

The water level must be dropped 2.5-5.1cm (1-2") from the top for two reasons; one, to allow for proper gas exchange and respiration of the frogs, and two, to prevent escape and therefore death. Otherwise ensure there are no escape holes for the frogs to explore. A DAF can easily jump 7.6-10.2cm (3-4") into the air from a solid base.

 

(very interesting source)

I've never heard of anyone keeping them in a tank without a cover before, simply because it's better to be safe than sorry, however on my research journey I did find this interesting tidbit:

The water level must be dropped 2.5-5.1cm (1-2") from the top for two reasons; one, to allow for proper gas exchange and respiration of the frogs, and two, to prevent escape and therefore death. Otherwise ensure there are no escape holes for the frogs to explore. A DAF can easily jump 7.6-10.2cm (3-4") into the air from a solid base.

(very interesting source)