If you have a one-dimensional cellular automaton (a single row of cells), and plot its states over time in successive rows, you end up with a two-dimensional picture, eg:

https://writings.stephenwolfram.com/2017/06/oh-my-gosh-its-c...

So you conceivably could generate mazes with a cellular automaton.

But Entombed doesn't. Figure 6 in the paper shows the geometry of the rule:

    c d e
  a b X

So it samples bits from the previous row, but also from the current row. In cellular automaton terms, the previous row is the previous timestep, but the current row is the current timestep, and a cellular automaton can't compute cells based on cells in the same timestep.