I think maybe this post would make more sense if you were familiar to its antecedent [0] which compares existing closure extensions to C. IIRC the comparison was in the context of considering designs for standardization.
C does not currently have closures, the post it looking at their performance properties with an eye for what form closures should be added to the standard.
There’s no necessary performance hit for closures. The performance cost here is caused by these closures needing to conform to a function pointer looking interface in order to be generally useful in C.
So, we need to swap to the logarithmic graphs to get a better picture
I wish more people would know about decibels.
C does not have closures. You could simulate closures, but it is neither robust not automatic compared to languages tha truly support them.
[0]: https://thephd.dev/the-cost-of-a-closure-in-c-c2y