Some thoughts:
My understanding is this is a powerful alkaloid. My experience has been that similar compounds can help combat inflammation. They also can work to help kill infection at times.
By the time people with Covid19 need oxygen, inflammation doesn't seem to be the primary problem. So I wouldn't think it would be particularly helpful at that point.
Ventilators get prescribed in part on the assumption that inflammation in the lungs is a major impediment to getting sufficient oxygen. Ventilators are failing at a shockingly high rate, with a death rate up around 80% or so. Some doctors are moving away from ventilators because the death rate is so high.
I spoke recently with a researcher who suggested impaired vasodilation was a possible explanation for the reports of low oxygen combined with a surprising lack of pneumonia and inflammation in some cases and that makes a lot of sense to me. The mechanism causing low oxygen does not appear to be the obvious answer of "lung inflammation and pneumonia."
That doesn't mean this drug can't play a meaningful role at some stage. It mostly means it's not the magic bullet solution for advanced cases that they were hoping for.
This in no way surprises me because the things it actually treats do not appear to be the cause of low oxygen in advanced covid19 cases.