He appears to be referring to the writing of benchmarks. Specifically for those in which functional programming excels over imperative (for example undo suppor

He appears to be referring to the writing of benchmarks. Specifically for those in which functional programming excels over imperative (for example undo support), it becomes a significant undertaking to write the C version so it just doesn't get done. This means that there is a selection bias towards benchmarks that can be easily written in C. These simpler benchmarks tend to lack any need for the higher-level constructs that functional programming provides, which FP advocates would argue is exactly where FP gives the greatest gain. edit: slightly less snark.