Three quick comments:

1) IMHO, the word fluent is used as part of an extended analogy. It is not meant literally, but since programming languages and natural languages have some similarity, it's not a complete analogy, which might be somewhat confusing. Keep in mind that Resnick is a professor at MIT and that Chomsky is also a professor at that same institution. Chomsky came up with the idea of "Generative Grammars" which is a concept taught in both linguistics and computer science. LINK: Generative grammar - Wikipedia, the free encyclopedia (Given that cultural background and set of shared assumptions, Resnick might see fluency in a way that is different from most English-speakers.)

2) Also IMHO, the term proficient (or highly proficient) could be substituted for fluent for the most part, but that would lose some of the meaning implied through the use of the linguistic analogy.

3) Also IMHO, Resnick's comments in the paragraph in question are in response mainly to the idea of what is commonly referred to as "Digital Natives." People born after the time that computers became common household items are sometimes called "digital natives" since they have never known a time when computers were not used in daily life. The erroneous assumption is that "digital natives" are automatically (by virtue of being born after a certain date and their exposure to technology) technologically proficient to the point of "fluency." Resnick argues that such proficiency should not be assumed. In fact, research shows that "digital natives" are no more technologically proficient on average than previous generations.