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I've followed Google pretty closely since 2001, and had somehow never encountered that term for matrix/spreadsheet.

I just had a hunch, opened http://sheets.new, opened the developer tools and did a CTRL+SHIFT+F search for "trix".

There were a bunch of CSS and JS hits. 791.

I think Trix is the internal codename for Sheets, like "Kix" is the codename for Docs (similar pile of matches for the term in devtools).