That is not how gases work. Anyone who took high school chemistry can shred the absurd reasoning this quack is pushing. Also he's a retired "nutritional researcher" so uh...
"In this study, researchers examined the blood oxygen levels in 53 surgeons using an oximeter. They measured blood oxygenation before surgery as well as at the end of surgeries.4 The researchers found that the mask reduced the blood oxygen levels (pa02) significantly. The longer the duration of wearing the mask, the greater the fall in blood oxygen levels." https://pubmed.ncbi.nlm.nih.gov/18500410/
""" Conclusions: Considering our findings, pulse rates of the surgeon's increase and SpO2 decrease after the first hour. This early change in SpO2 may be either due to the facial mask or the operational stress. Since a very small decrease in saturation at this level, reflects a large decrease in PaO2, our findings may have a clinical value for the health workers and the surgeon """
Assuming the oxygenation levels are down because of the mask and not stress influencing your breathing (let's abandon occam's razor for now [0]), this "after the first hour" bit can tell you that maybe the word "significant" just mean that this is measurable and not dangerous? i mean if you drop from 97.5 to 95 over 4 hours, this is significant. Will that have an impact on you?
Moreover blood oxygenation do not drop linearly. If your body consider the oxygenation too low, your pulse will quicken, you will breath harder. Your body will try to keep your oxygen level high harder and harder.
This study only concerned surgeons who are often healthier than the average population. Would this have been done on people with lung issues with oxygen in low 90s high 80s, yes this would have been concerning.
Again ignoring the stress factor. And as long as the most plausible explanation is not overruled, it should prevail as any rational person will tell you. So this premise is false, or at least not proven (yet) and therefore even if your argument was good, the conclusion can be false.
[0] https://pubmed.ncbi.nlm.nih.gov/12931769/ (first link i found on pubmed)