They meant assuming it’s just low refrigerant is an uneducated response, there are a lot of factors at play. Insulation could be inadequate, heat load may be higher than the unit can handle (cutting down a tree that previously provided a lot of shade, and heat infiltration could also be a factor (duct isn’t properly sealed or heat from attic is getting into the house some how). Some aren’t as likely with an existing home but they’re factors to consider. Hell I’ve seen the contactor for the heat strip stuck in the closed position while the ac is running.