This is the meter used in the video:
Note that the sensor is situated at the top, pointing upward. When the person in the video breathes out, the exhaled air will be directed into the sensor, but when the person inhales air will flow around the sensor with no strong tendency to enter it.
This creates a strong bias for measuring exhaled air vs inhaled air, and likely measurements would not be radically different if the same experiment were conducted with no mask present.
In one small study published in Respiratory Physiology and
Neurobiology, twenty subjects wearing surgical masks walked on
treadmills for one hour. Scientists measured their blood oxygen and
carbon dioxide concentrations, respiratory and heart rates, and core
temperatures. After that hour, the scientists found no significant
change in these measurements.
With N95 masks, it’s a slightly different story. There is some
evidence that these masks, which tend to fit more tightly over the
wearers face, can decrease oxygen levels and increase carbon dioxide
levels. A small study of ten healthcare workers found that the oxygen
and carbon dioxide concentrations within N95 masks fell below
workplace standards set by the Occupational Safety and Health
Administration. Still, when the scientists compared the subjects’
blood oxygen levels after one hour on the treadmill in an N95 mask
versus an hour without, they found no difference. Another study
published in the American Journal of Infection Control found similar
results with pregnant study subjects — after an hour of walking in N95
masks, the blood oxygen levels of pregnant women and non-pregnant
women alike hadn’t changed. The masks also had no apparent effect on
the fetuses, whose heart rate didn’t change throughout the study.