The code doesn't actually say it, but the handbook does.
At the end of the paragraph in 310.4, the handbook states (I know this then isn't code)
Conductors connected in parallel, in accordance with 310.4, are considered a single conductor with a total cross-sectional area of all conductors in parallel.
So if I am reading this right, let’s say we have a 400A run. We could run 500 kcmil (380A second column, but round up to next breaker size).
Let’s instead run this in parallel, according to the note in the handbook, running in parallel is equivalent to adding the cross-sectional area.
By table 5:
500 kcmil area = 456.3 mm^2
Finding a wire size that is at least half that would be:
250 kcmil area =256.1 mm^2 * 2 = 512.2 mm^2
4/0 is less that the area of 500 kcmil.
On lots of projects, its called out to parallel 2 3/0s to get 400A. This seems wrong to me if this note is correct.