Yes, they will be able to quickly pickup related languages with relative ease just as someone who knows a romance language will be able to pickup other romance languages without much difficulty. But that person who knows C++ and Pynthon with nothing of the underlying workings will have a more difficult time learning assembly, Forth or Lisp than a related language just as the person who knows a romance language will have a more difficult time learning Swahili than another romance language. Not a non-sequitor at all. When you look at CS or linguistics and how they relate to the languages they deal with what they really are are the study of what those languages have in common and understanding that will decrease what you need to learn when learning new languages.
Understanding a language and using a language are very different things and this is why toddlers can pickup languages with more ease than others, they do not care so much about understanding, and this is the primary limitation of our first language(s) and why being bilingual offers little cognitive advantage for most people.