I was mistaken, I found a copy of Jobs' letter re:Flash and he does cite the proprietary nature of Flash and Apple's lack of control over the content served on its platform as the most important reason Flash was kept off the iPhone.
https://www.cnet.com/culture/steve-jobs-letter-explaining-ap...
As hard as it was to believe: Flash had been responsible for every single beachball that week. Yikes.
I used OSX and Flash in the late 2000s so I have no problem believing that.