Neither the OT nor the EU were butchered to conform to the Holiday Special or include “holidayisms”, so your argument is invalid.
- Bea Arthur is canonical now: http://www.ign.com/articles/2017/10/24/a-character-from-the-star-wars-holiday-special-is-canon-now
That’s not in the OT.
- Death of Chief Bast aboard the Death Star needs to be reconsidered, as he appeared in SWHS a year later.
That’s not in the OT.
- Kashyyyk, as shown in SWHS (and drawn by Ralph McQuarrie), eventually appeared in ROTS.
That’s not in the OT.
- According to “The Visual Dictionary of Star Wars, Episode III”, Itchy appeared in ROTS.
That’s not in the OT.
- Lumpa appeared in several EU literature, and Waroo is a canonical character since Aftermath: Empire’s End.
That’s not in the OT.
- many other references to SWHS have been made in the EU (see https://en.wikipedia.org/wiki/Star_Wars_Holiday_Special#Canonicity_of_the_special, “later appearances”).
That’s not in the OT.
So it seems that both OT and EU include several mentions to SWHS
None of that was in the OT.