I interpreted Comed-tea as the simplest example of backwards causality - an event A causing event B, where A occurs /after/ B in time. Eliezer introduced Comed-Tea to make the point that the HPMoR universe does not operate by what we imagine to be standard causality rules.

I suspect that, the same way that messing with Time somehow results in a message saying "NO", it would be similarly impossible to commit to drinking Comed Tea.