I think they are saying that generally every day sells out, and therefore, no Saturdays are no busy than any other day of the week.
I personally think that maybe during the off season, perhaps the weekdays will be a little less crowded than weekends. Generally, in the off season they offer some really good Florida Resident's Discounts then to try and get people to come when they are slow, and locals will be more likely to go on Saturday or Sundays. So perhaps then, yes when they are not selling out, they might be less busy.
Either way, the way Discovery Cove was designed, was so guests wouldn't be worried about crowd levels, because it's never going to be very crowded.