This way around isn't as fatal as putting IIci cards into an SE/30 slot though.
While both directions will totally not work at all a IIci card in an SE/30 slot will be damaged because the SE/30 slot has 12V present in places where only 5 logic level signals should go.
Putting an SE/30 card into the IIci slot will just short 5V power supply pins to ground causing the power supply to shut down. It's less likely that something will actually get damaged because there is no 12V present on the IIci slot at least.