Blood oxygen of 89 isn’t all that bad. A lot of people with COPD are that low much of the time.
Until I got my CPAP I would often be mid-80s during an apnea episode. Not good, but not immediately life-threatening either.
People with COPD with blood po2 of 89 have had years to compensate with increased red cells. Someone who runs normally in the high 90s who drops 10 points or so is severely compromised.