Heo Jin-ho is not a historical film director, but a director who writes novels about historical figures

First of all, he picks a historical figure he wants to write about, stuffs them with all the attributes he wants to include to create a new character, and then calls it "historical reinterpretation."
He probably thought Princess Deokhye would have some recognition thanks to novels, and since the idea of "Princess Deokhye being a leader of Koreans in Japan and an independence activist" fits perfectly with anti-Japanese nationalism, he used it... He could have highlighted actual independence activists, but if he had done that...
The real Princess Deokhye had been suffering from schizophrenia since her teens. The director had no interest in the actual "Princess Deokhye"; instead, he created a version of her that could attract public interest while allowing him to attach whatever attributes he desired.
While Lee Joon-ik focused on researching and implementing the life of Crown Prince Sado, Heo Jin-ho took the doll called Princess Deokhye, plastered it with the attributes he wanted, and promoted it as a "historical film." It might have been different if he had emphasized that it was a "faction" (fact + fiction), but he promoted it as if it were a moving true story.
It's not as obvious with Princess Deokhye or Forbidden Dream because they are set in the Joseon era or the Japanese colonial period, but he does the exact same thing with modern history;;
Coincidentally, critic Lee Dong-jin also gave Princess Deokhye three stars. The production company is also the same, Hive Media Corp.
He was criticized even back then for historical distortion. At this point, it's just the director's habit. He doesn't even realize this is historical distortion;;
"Who would empathize with such absurd glorification of the royal family?" Entermedia "Princess Deokhye," a strange double play depicting the Joseon royal family [Entermedia = Hwang Jin-mi's Biased Review] ▲Anti-this movie▲. (The text contains spoilers. Please be advised if you plan to watch the film.) is a film depicting the life of a real person known as the "last princess of Joseon," and is currently a hit with over 4 million viewers.