You can do extreme duty cycles either way (on time or off time) with diode(s) parallel to timing resistor, xyz-prefix 555 pwm-ing via control pin is - as far as I remember - limited to pretty much same as why it has a 5-5-5k divider inside: 1/3rd to 2/3rds vcc fiddling range. Do check for yourself, just in case.
If you have that as a tht IC, ffs man, put it on a breadboard and see when Vout becomes an unacceptable sludge of a signal or of woeful amplitude for triggering ffs and logic downstream with an oscilloscope and a trimpot on pin 5. NXP's test set-up isn't what you'll do with it, after all.
I think 555 fmax should be interpreted similar to OA or any other device's fmax (much less than might be hoped for from what top of a datasheet claims, and depends on circuit used in, etc.).