UA's analogy is a little weak, and I wouldn't call it "throwing shade," but they really should stay above the fray and simply provide the logical reasons why E+ empties are not available after takeoff. Perhaps with a positive statement that if the pax really wants an E+ empty seat, to be sure to flag down a flight attendant so they can collect the additional fee.
A better analogy is: An empty first class seat is obviously not available free after takeoff. The same policy applies for the Economy Plus section.