Even if the requirement is "first food or beverage" instead of just being seated at a table, the time between being seated and getting a glass of water (especially in Florida) is likely a matter of minutes. How much of a difference do you believe that actually makes, especially if you're not interacting with anyone? From the reports I've been seeing, at least at table service restaurants, people are taking off masks when they are seated and the servers are all fine with it. At one point, the rule in my state was that you had to put on a mask whenever interacting with servers even if you had food and drinks on the table. Disney is not requiring this.
Like all rule changes at Disney, I suspect this one was adopted to address a specific problem, and I don't think the problem was having people go without a mask for the couple of minutes between being seated and getting water. More likely they are thinking of people waiting in line or sitting somewhere waiting to be seated.