Your assumption appears to be suggested also here:
Apparently a blending of step with set foot, perhaps by confusion.
(chiefly US) Alternative form of set foot:
An early usage is from the the beginning of the 19th century:
1813, Washington Irving, “Sketches of an Excursion from Edinburgh to Dublin”, in The Analectic magazine, page 480:
- This was a pleasure of no small kind; and in stepping foot again upon the soil of that country, which contains much that I prize, and more that I admire.
And also the in the following extract the Washington State University confirms that step foot is a misusage of set foot:
- When you want to say that you refuse to enter some location, the traditional expression is not “step foot,” but “set foot”: “I refuse to set foot in my brother-in-law’s house while he lets his vicious pit bull run around inside.”