These two sentences seem contradictory from the author's response is contradictory: " The IRB of UMN reviewed the study and determined that this is not human research (a formal IRB exempt letter was obtained). Throughout the study, we honestly did not think this is human research, so we did not apply for an IRB approval in the beginning."

I would guess their IRB had a quick sanity check process to ensure there was no human subject research in the experiment. This is actually a good thing if scientists use their ethics and apply good judgement. Now, whoever makes that determination does so based on initial documentation supplied by the researchers. If so, the researchers should show what they submitted to get the exemption.

Again, the implication is their University will likely make it harder to get exemptions after this fiasco. This mistake hurts everyone (be it indirectly). Although, and this is being quite facetious and macabre, the researchers have inadvertently exposed a bug in their own institutions IRB process!