While it's a charming piece of folklore, the real origin of the word is more linguistic than royal:

Although the name predates the kings in the legends, the " Sir Loin " pun was a popular joke in royal courts as early as the 1630s. Why It's Worthy of Legend

As the story goes, a British King—often identified as or Henry VIII —was enjoying a particularly magnificent feast. He was so impressed by the quality, tenderness, and flavor of a specific cut of beef loin that he supposedly drew his sword in a fit of joy.

The most famous "story" regarding the beef loin is the legendary tale of how the earned its name through a royal knighthood. The Legend of Sir Loin

The word "sirloin" actually comes from the Old French word surloigne .

"Sur" means "above" and "loigne" means "loin," referring to the cut's physical location on the animal— above the loin .

He tapped the meat and declared, —symbolically knighting the steak for its superior taste. The Truth Behind the Name