Excel 365 SUM dynamic array at row level

Lorenzo 

Since you now have Lambda functions, the solution simplifies.  First, assuming the dummy array has been given a name

= BYROW(Dummy_arr, Sumλ)

where

Sumλ = LAMBDA(x,SUM(x))

or including the array definition within a LET

= LET(
      dummyArr, {1;10;100}*SEQUENCE(1,3),
      BYROW(dummyArr, Sumλ)
  )

I have attached a file, but it contains little by way of additional information.