I'm pretty sure in an old thread on here about microbial corrosion, evidence was posted to show that yes, the bugs do actually directly eat the steel.
I remain to be convinced though. On the boat I sold a year or two ago, on its first blacking after I bought it back in about 1997, I remember the hull having lots of rust patches about the size of a 10p coin and these fell off readily, leaving a bright shiny silver patch of steel exposed. Sounds exactly like the descriptions on here of microbial corrosion.
Anyway I asked Mike Butcher about them and he said they are common and not to worry, he just blacks straight over them and it will be fine. Which it was. Boat had a 100% A1 hull survey about six or seven years ago, and another just after I sold it, equally good.
No sign of the same silver patches on the subsequent blackings I did.
Just a data point of one.