CambridgeGEL, page 1588 reads
Examples like She’s six foot tall involve a special use of the singular form rather than a base plural: the difference between this and How many feet are there in a mile? is a matter of syntax rather than of inflectional morphology.
In a herd of elephant it is arguable that the construction involves not a base plural, but a special use of the singular in certain syntactic contexts (comparable to the six foot tall construction).
What are that "special use" and those "syntactic contexts" the author refers to?