Just to clarify: the incubation period fits like a glove for the Rose Garden scenario: expoure on Saturday afternoon, first symptoms on Thursday night = 5 days (and a couple of hours).
However, what is alarming is the very fast progression of symptoms: Typically, symptomatic Covid-19 develops more slowly. People tend to have relatively mild symptoms for about a week (1st phase of the illness), and on around day 7, they either overcome it and never enter phase 2, or they progress, rather suddenly, into phase 2, and deteriorate into requiring oxygen and general hospital care. It's just how the virus works. Yes, we do know that POTUS is a little old (but biologically much younger) and a bit overweight, but that's it. There is absolutely no reason that he should be feeling a bit "rough" on Thursday evening, and going down so quickly as to being in need of supplemental oxygen on Friday.
Possibilities: he was either exposed to a very large dose, and/or the virus he got infected with is more weaponized and thus behaves slightly differently. Which would also explain why it was not picked up despite daily testing (different genetic sequence can throw the test off).
It would be interesting to know exactly how the other infected people are doing and if they have a similarly abnormal course. Might be worth keeping track, if possible.
We should treat this as an assassination attempt and above all: save and archive all evidence we can get our hands on, such as any and all Rose Garden photos and videos.