20 30 ∘.+ 0 ∘.+ 3 4 5 6
23 24 25 26
33 34 35 36

The scalar 0 is considered an array, with empty shape!

      ⍴0  ⍝ Dyalog isn't talking to me

In order to preserve the property ×/a,b ←→ (×/a)×(×/b), ×/ must send the identity ⍬ for , to the identity 1 for ×.

      ×/⍬
1

APL knows!