Undoubtedly, what John Walton would be referring to are the cantillation marks, also called the Te'amim, which were added by the Masoretes along with the niqqud (vowel pointings).
The Te'amim are a complex system of marks that serve as punctuation and musical notation at the same time. For a brief introduction into their function, one may enjoy watching this MINI LESSON on youtube, presented by a Jewish professor of Hebrew.
Within the Te'amim system, marks provide indication of where the clauses of the sentence are to be divided, what is the most important word in each, which clause is of greater emphasis, where word accentuation should be, and more. Essentially, they serve as the equivalent to English commas, periods, colons, semicolons, quotation marks, exclamation points, and more even than English has available.
With the Hebrew punctuation, setting apart one clause or expression of a sentence could dissociate it from a later declaration. With respect to the verse in question--Genesis 1:21, it could be understood as punctuated something like the following:
So God created great sea creatures: And every living thing that moves,
with which the waters abounded, according to their kind; and every
winged bird according to its kind and God saw that it was good.
The "half-way point" marking, similar to a caret under the word (הַגְּדֹלִ֑ים/hag·gə·ḏō·lîm), indicates the point at which the two "halves" of the sentence should be split--which has nothing to do with actual length and everything to do with meaning. This is the word "great" (which comes after the word translated as "sea creatures"--remember, Hebrew reads right-to-left, and in Hebrew, adjectives usually follow their nouns), which means that the "great sea creatures" is the end of the first "half" of this verse. Everything past that expression is part of a second major clause to the verse. Further, there is no division made, in the Hebrew, between "according to its kind" and "and God saw that it was good." This links, without separation, these two clauses, which can be interpreted as part of a unit.
But the Masoretes may have, without realizing it, parsed it this way on account of the fact that the "תַּנִּינִ֖ם/tan·nî·nim" represent forces of evil that were to exist for a designated time in earth's history.