They say it's due to lower orbits. Iridium is fewer satellites so they have to be higher. More w/ lower orbits means lower latency. Do you know if this is too good to be true?
40000 / pi / 2 = 6366 40000 / 2 * 3 = 60000 60000 / pi / 2 = 9550 9550 - 6366 = 3184
Per the above, any satellite Internet system below about 3000 km above the earth will have latency that's competitive with fiber optics on earth (note that I haven't done the simultaneous solution to account for the uplink/downlink distance so it's actually a bit less). For the SpaceX system, there should be better latency than the terrestrial Internet. Please ignore the rounding ... this is a back-of-the-envelope exercise.
Fiber optics are great and far less susceptible to interference than radio - but if you Google articles on how high-frequency trading companies are installing line-of-sight microwave for their networks you'll see that fiber isn't our fastest technology.