well, thermo is from the greek word thérmē meaning heat. we use the termo- suffix for all things that involve heat (thermodynamics -> termodinámica, thermometer -> termómetro)
I am aware of the etymology you refer to, but in this case I am referring to something different.
The noun "termo" in Spanish is formally recognized by the RAE to refer to a vacuum insulated food jar (such as the ones sold by Thermos®), and that is the word I was referring to, as it is the most widely identifiable use of the word "termo" in Spanish.
