they start wiggling around, and the friction generates plenty of heat.
The process of their starting to wiggle around is already heat. Friction of molecules rubbing against each other is not required. That they are not all wiggling in near-perfect synchrony is enough. That could maybe be considered kind of friction-like, but A) that’s not what we normally think about when we say ‘friction’, so it’s misleading to use it for that, and B) it doesn’t happen after the wiggling around or in response to it; it’s just the way the material reacts to that kind of radiation.
Now, if it were long-wave radio, that would be a lot more like what you describe—you’d get electrical currents, and those would produce heat by friction. But just making water molecules reorient is a direct heat-generating activity.
The process of their starting to wiggle around is already heat. Friction of molecules rubbing against each other is not required. That they are not all wiggling in near-perfect synchrony is enough. That could maybe be considered kind of friction-like, but A) that’s not what we normally think about when we say ‘friction’, so it’s misleading to use it for that, and B) it doesn’t happen after the wiggling around or in response to it; it’s just the way the material reacts to that kind of radiation.
Now, if it were long-wave radio, that would be a lot more like what you describe—you’d get electrical currents, and those would produce heat by friction. But just making water molecules reorient is a direct heat-generating activity.