"The Smurf Who Would Be King" is the only episode where Smurfette doesn't physically appear but appears only in a vision in Dreamy's presumed dream.
This episode makes reference to three others: "The Astrosmurf" from Season 1, and "The Last Smurfberry" and "Handy's Kite" from earlier in Season 3.
Dreamy's voice is notably higher in this episode, resembling his voice in "Dreamy's Nightmare" from Season 1. This is the final episode to feature his high-pitched voice.
The title "The Smurf Who Would Be King" is an allusion to the Rudyard Kipling short story "The Man Who Would Be King", which became a 1975 film.
As to whether Dreamy's adventure actually happened, the only possible explanation comes from the non-canonical Smurf film The Smurfs and the Magic Flute (1976), where Enchanter Homnibus uses a technique called "hypno-kinesis" which causes Johan and Peewit to magically appear near the Smurf Village while also still being in Homnibus' hovel. But, since in this story there is no wizard that causes Dreamy to fall asleep and appear in the land of the Pookies, this explanation is uncertain at best.