I'm unfortunately not there physically to help, but my mom has this light switch she needs to remove temporarily and replace the plate. Her "handyman" installed it 'brand new' within the past few years. I told her it looked like it was from the 80s haha. Anyways, she apparently cannot remove the knob for the life of her, shouldn't the knob be able to simply pull off without breaking?
That style should be clutch fit, i.e. the knob just pulls off. It may be gunked on there with 40 years of accumulated crud, though. So this is likely to require a hard yank or possibly some prying. Good news: The cover plate is already broken, so prying on it can't possibly meaningfully break it further.
It's also not outside the realm of possibility that her handyman broke it, and his solution for "fixing" it was to just glue the knob onto its stem before anybody noticed.
If it breaks, no big deal. Breaking it is also a valid way to get it off, and an entire replacement dimmer is like $9 at the hardware store. You can also get replacement knobs for a couple of bucks, and they're generally broadly interchangeable (although these days, without that groovy aluminum accent disk in the middle).