I agree with you. I have heard that other argument and seen it presented in print as fact but I believe it is a misreading of the regulations, an assumption arrising from the fact that to be labeled "corn" the whiskey must be at least 80% corn, so the reader extrapolates and assumes that "bourbon whiskey" becomes "corn whiskey" at that 80% threshold, but you don't read laws that way. They only say what they say and you can only draw inferences when they are inescapable. The regs say nothing about a maximum percentage for the principal grain nor about a minimum percentage for the secondary grains nor, in fact, anything about the secondary grains.
I have "discussed" this with some high ranking officials from other distilleries and they have maintained that 80% is the top limit on corn; I have maintained that according to the regs, you can use 81% up to 100% and still call it bourbon (providing the rest of the regs are followed). Corn whiskey is basically a euphemism for product aged in used cooperage.