Thank you for noticing this. Yes, the CACE can be negative. It's just a risk difference and the sign will depend on which group you subtract from the other.
I've changed Slide 24 to appropriately label the CACE on that slide as a risk REDUCTION and added that clarification to the derivation in the speaker notes.
I've uploaded the new version of the slides with this correction and today's date as part of the filename to the CLE.
Best wishes,
Tom