"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.