It's likely that Harry's assessment of how Comed-Tea works isn't airtight. At the very least, it's certainly not the only possible explanation. The other comments posted so far have already produced more than one solid alternative.

Most importantly, there's no particular reason to suspect that the Tea relies on one thing alone. If it could be prescience + mind control, why not prescience + mind control + occasional bad taste? Harry's current top theory of prescience + mind control, for one, is already more complicated than it needs to be; we could easily cut out the prescience part and presume that the Tea works by merely lowering the drinker's threshold for choking on it/spitting it out.