I contend there is a considerable difference between your original statement
"Carbon dioxide is a liquid at room temperature (otherwise CO2 fire
extinguishers wouldn't work)." and your current statement "CO2 fire
extinguishers do indeed contain liquid CO2 at room temperature." Since your
original statement contained no restrictive modifiers, its implied message
was that CO2 is a liquid at room temperature in general, not just in fire
extinguishers.
I totally agree with you that physics teachers should strive for unambiguous
speech. I think you missed the goal in your original statement.