Are HVAC systems responsible for SARS-CoV-2 diffusion as they are for the spread of the Legionella bacteria?

NO. Legionella is a bacterium which can grow naturally in fresh water as well as in some of the HVAC equipment where water is used and collected (humidification AHU sections, cooling towers, etc.). It is usually spread by breathing in mist generated in cooling towers, though it may be found in other HVAC equipment where water has accumulated. The bacteria can eventually enter into the building through air-conditioned infected air.

On the other hand, SARS-CoV2 is a virus that can live only inside humans and infection occurs only via human-to-human transmission.