Am I right in thinking that formula fields are not supported by GetCell? They don’t seem to be for me, at least. (I get the “column does not exist” error, even though it’s visible and populated on the view in question.)
You are right Spreadsheet does not support formulas. Formulas can be defined in the AirTable project itself or implemented algorithmically in the Thunkable X project. If desired, you can even make a parser of formulas.