Medicine is very rarely about absolutes, it's mostly about how things get better or worse. When I was in the hospital, no one looked at the absolute values of my blood work, they cared to see if it dropped based on the treatment they saw.
Also, doctors are already aware about how oximeters may be inaccurate. It doesn't mean that they still can't be useful.
Yeah, if we had such cut and dry "rules" about medicine, then we could automate a lot of what doctors do. Medicine is as much about the art as it is the science. You come to understand the limitations of different measuring techniques, and learn to know when you need to rely on each for more accurate representations.
Also, doctors are already aware about how oximeters may be inaccurate. It doesn't mean that they still can't be useful.