If you had read through the features, the ability to limit backorders is a pro feature, in fact, it is the first feature.
EXTRA PRO FEATURES
Set the maximum allowed number of backorders for a product and product variation.
Only $25, but for a third-party plugin with no way to verify support or functionality before purchase, I would rather have this be a first-party feature.
Especially, considering how simple of a feature this seems to be? It’s so basic that it’s very confusing why it’s not included.