"Factual mistake" is a bit harsh, but the missing piece is that there are multiple ways to romanise Japanese; all of them produce "valid Romaji" but only in the particular system being used. Si is how you write し in romaji using the kunrei-shiki romanisation. In the Hepburn romanisation it's shi. See https://en.wikipedia.org/wiki/Romanization_of_Japanese#Diffe... .
I see, maybe my use of “romaji” is sloppy because I implied “Hepburn romaji” specifically because that’s what I chose in the article. I do explicitly mention that other romanizations exist and that I chose not to use them — so I’m not worried about misleading someone who reads the article. But on pedantic level I see why “si doesn’t exist” sounds overly broad.
Kunrei has been deprecated, Hepburn is the official one even the government of Japan recommends these days and matches better the pronunciation of the language.