[throwaway284927]

To me, they both seem to be talking about what explains the perceived difference in effectiveness and efficiency between children and adults when they learn a new language.

The first comment seems to be arguing that the difference is "literally just a function of time and exposure opportunity and nothing else" (explicitly ruling out a significant effect related to a difference in brain plasticity related to age), while the comment you're answering to argues that brain plasticity, even under the same conditions of time and exposure opportunity, makes a significant difference.