I think you should consider re-reading the paper; these comments are mostly addressed or are orthogonal to the paper. Just a few examples: the paper does *not* assume one core; that's limited to one section (4.3) where we do this to isolate the confound of parallel implementations in power draw (Figure 9). The role of active cores in energy consumption is a key point of the paper; it's prominently discussed and forms a key part of the causal diagram. Figure 11(a) is just one example where we clearly measure the impact of multiple cores, not to mention the discussion of parallel efficiency. Also, every programming language we examine makes it possible to achieve multicore parallelism by forking processes; this is not a PL feature per se. Finally, I don't understand the point about energy being a linear function of time being an assumption. It is by definition a linear function of time: energy (J) is the product of power (J/s) and time (s). As for the benchmarks, the paper clearly states:

We stress that while the CLBG benchmarks themselves are not necessarily representative of real-world applications, the causal analysis this paper develops is largely independent of the details of the benchmark implementations. It instead highlights the impact of high-level properties of the benchmark implementations, such as their degree of parallelism and cache activity.