There's no deal. 6 is the highest level for dust protection. It means “No ingress of dust; complete protection against contact (dust-tight). A vacuum must be applied. Test duration of up to 8 hours based on airflow.” It means an airtight device. There's no dust protection above that. If water, the molecule, can't get in or harm a phone, no particle will get in.
EDIT: Just realized I didn't clear up this point well enough. You got it the other way around. Keeping dust out is easy, water is hard. Because with water you have the factor of pressure defeating seals and leaking in. Something that is less likely to happen with a gas forcing particulates in. There's no natural atmospheric pressure on earth that could defeat an airtight seal enough to force dust in. But on the contrary, water pressure that can easily crush phones is actually a quite common occurrence. And enough pressure to break seals and leak water in can happen in only a few meters of water submersion.
Atmospheric dust is anything between 0.001 and 40 microns. Typical rice starch (what rice dust is made of) is around 0.5 microns, the smallest it can get is 0.1 micron.
Rice particulates do nothing to phones. Apple just wants people to pay for a new phone or Apple's overpriced tech service.
ADD: As stated by the article the rice hack doesn't work anyway, it's just a meme. But the rice won't hurt an IP68 phone.