In principle every notebook is a DAG, since the cells are evaluated in a certain order. You could even infer the DAG from Jupyter notebooks, at least when the execution number is stored (the In[...] part).

There is a lot of value in imposing restrictions on the cells (like no reallocation of global variables) to be able to infer the DAG more directly like they do here (or Pluto.jl does). It is simpler, it can serve a dual purpose by being a normal runnable python/Julia/whatever program. Another advantage that I didn't see mentioned in TFA is parallelism. There is some interesting potential there I think.