One reason could be your water has just an higher oxygen level. (depending on how flexible your world is).
Also if I understand the video correct it assumes that you breath out much of your oxygen for calculating the 100 liters of water. You could get much lower if you calculate with the value that is used by the body. Following numbers I use came from the German Wikipedia.
A adult human takes between 11 and 15 breaths per minute and has a lung volume of 0.5 l.
21% of the air we breath in are oxygen, 17% we breathe out. So we use 4% of the air we breath as oxygen. That is 220 ml to 300 ml.
If I take the oxygen level of water from the video (0.01 l oxygen per l water down to 0.005) that means if I could get all the oxygen from the water I only need 22 (best case) to 60 (worst case) l of water per minute.
Best case is higher oxygen level with lower consumption, worst case the lower level with higher demand.
In another thread for mermaid it was proposed that the long hair could be used for extracting oxygen. Using such big and wide spread organ to absorb oxygen would increase the water that can be used.
The last thing I could imagine would be a lower metabolism while in water. If they can use their lungs there energy demand will get higher, their body temperature rises and maybe even getting more intelligent.
