The ability to recall one or two 3 or 4 digit numbers having previously "jotted" them down plays well here. If you can remember the 900 in this worked example, whilst doing the other half of the sum.. you hit a moment where the heuristic recurses and you have to "jot down" a partial result to finish the sub calculation.. to then add to the damn.. what was that value again...
I prefer aphantastic to aphantasiac. I consider it a more fantastic term.
As an aphantastic, sub vocalizing as described is exactly how I do mental math.