I think you've confused what mutable state is. The cell values entered by a user are input values. The derived value of a cell bases on the computation of other cells, which is not changed once computed. When a user changes an input value in a cell, a new round of computation of the whole spreadsheet is started. The input value are not mutable state during computation. The modal of computation of a spreadsheet is pretty much stateless.

The computation between cells are done by the declaring formulas and cell dependency. The order of computation or how the computation is carried out is not specified, which is what declarative programming is.

The input cells and derived cells are pretty much like the tables and computed views in a RDBMS, where SQL is a declarative language to define the view.