Isn't Honi supposed to be written Honni?

Well, yes and no. In 1348, the spelling was honi. But, like most languages, French evolved and today the double N is the norm in the French language. Henriette Walter wrote a book documenting the 'love story' between the French and English languages. It is aptly named "Honni soit qui mal y pense" (note one of two 'n' is italicized)

Source: interview with Henriette Walter