The 30ft per millibar difference is only valid up to (very approx) 20 - 23,000ish ft. At 35,000ft the difference can easily be 70ft per millibar!
What you write above is quite correct, but it's not the point. The level difference between the 250 mbar level and the 249 mbar level is actually about 100 ft. But that's irrelevant when it comes to separating traffic on different altimeter settings. If you change the altimeter subscale setting by 1 mbar, the difference in altitude indicated by the altimeter is 30 ft, because you're changing the reference level by about 30 ft, and the altimeter simply measures the altitude difference from that reference level.
Try it out at FL350 some time.