Yeah he is wrong. Well at least for cannabis. Maybe is truth for other plants tough...
As for cannabis, if you do little bit of research around forums you can see most peoples tells 78 is the optimal. I do NOT consider you to keep your water under 70, by experience i learned that my plants growth slower under 70 degree C. 78 might be the optimal but it cost a lots more of nutes that way because of the water evaporation... so i put mine at 72-74 to avoid more expenses...
Anyway i hope you find the best temp for your needs.