I feel like you’re going out of your way to misinterpret the article. As the article says below:

> this is why it's important that you don't actually "think in" romaji. i'm using romaji as a convenient way to refer to phonetics in text. however, your "mental algebra" should match the hiragana table.

Then the article includes an exercise that verifies the reader’s understanding.

I also included a note:

> (note i could also have used a different romanization that renders し as "si", つ as "tu", and ち as "ti" for this article. i decided to not because everyone else uses romaji, and once you understand this point once, you shouldn't have a difficulty doing this in your head.)

Where is the factual mistake here? “si” is invalid romaji, my article uses romaji, therefore it’s invalid.