According to this article, in the ISDA SIMM methodology, the delta sensitivity for interest rate products is calculated using
$$ \text{delta} = \frac{V (x + 1 \text{bp}) - V (x - 1 \text{bp})}{2} $$
However, when I look into the ISDA SIMM 2.5 documentation, I fail to find any reference to the formula above. Could you please opine what is the basis of this delta calculation?