The water from the soda dispenser is filtered as much as the water used in the soda -the inlet connection to the machine or any locally mounted filter would almost always be the same. To what degree the water is filtered is a difficult question to answer. Most machines use a fine sediment filter(< or = 5 microns) to avoid getting stuff in the machines/tubing/valves. Some places may use as much as a three stage (including a carbon filter to remove certain organics and taste/odor) and filter down to 1/2 a micron, but this is quite rare. At this level(1/2 micron) most all parasites such as giardia will be removed, but you have to filter to .2 microns(you won't see this level except with expensive small ceramic drinking water filters) to get rid of bacteria, and viruses are pretty much impossible to stop. Most bacteria and viruses are killed back during the chemical treatment stage in the water system. I highly doubt WDW does anything beyond normal sediment filtering. Not that it matters much - most water in Florida tastes like skunkp***. It's either over-clorinated to kill the pathogens, tastes "salty" due to hardness, or if from a deep well, loaded with sulfur and/or iron. Edit: Reedy Creek, aka WDW, gets all its water from 12 wells on the property.
Also remember that someone changing a filter can easily contaminate the downstream(clean) side, and that filters can leak or be misinstalled rendering them ineffective. Add to that any ice may not have similar filtering and that ice machines are a historically a great place to find slime, bacteria, and more.
