The Japanese phonological system doesn't allow a /s/ sound to occur before the vowel /i/, the consonant must undergo palatalization and become /ɕ/ (the IPA symbol for the Japanese sh-like sound). Because this is a regular sound rule, the native writing system doesn't have a way to distinguish the nonexistent */si/ sequence from the /ɕi/ sequence that actually occurs, and this is the syllable that hiragana し or katakana シ indicate.

In the Hepburn romanization system, which generally tries to be transparent to speakers of English or other European languages, し is romanized as _shi_, because this indicates to English speakers that the /s/ -> /ɕ/ sound change happens. In the Kunrei-siki romanization system, which tries to be more faithful to the distinctions made in the Japanese phonological system, し is romanized as _si_ to be consistent with the other possible syllables _sa_ _su_ _se_ _so_ that begin with the consonant /s/.

And yeah the fact that the article-writer hasn't internalized this sound change yet is a sign that their command of Japanese isn't all that good yet.