Pre-defined assets can be used with any generic capability. I am not sure why it seems that you need to specify the text label “Medium” in order for that given preset to work other than the way you are requesting it.
As far as your item configuration, I would recommend using the new metadata syntax by simply setting the metadata value to Fan without any additional parameters. This will have the same effect than how you have it configured right now. Make sure to trigger a discovery once you updated your configuration.
It is interesting that a Dimmer item is used to control the fan speed in steps. Usually fan controller dimmers are used in a more linear way.
Anyway, it would make more sense to use a proxy Number or String item in this case that would be exposed to Alexa and control your Dimmer item in steps through rules. This should make the Alexa app integration much easier for that device as well.
String FanMode "Fan Mode [%s]" {alexa="Fan"}
Otherwise, as an undocumented configuration, you can specify the presets parameter on the Dimmer item. This will override the presets defined by the FanSpeed attribute.
