The time taken to count out sets simply consumes the instructional time available. The last sentence is irrelevant to the purpose of teaching counting objects. It violates the principle of Abstraction. Since it attempts to justify the preceding statement, the latter also becomes suspect. Indeed, later on the same page we read Counting objects, used extensively to teach small whole-number concepts, are also used to introduce grouping.
Children count sets, grouping the objects in sets of ten.
They can quickly learn that it is easier to count sets grouped by tens and then whatever loose ones may be left over than it is to count by ones. They can learn to associate the tens and the ones with the spoken and written forms of the numeral. Which, in conflict with the first quote, appears to suggest a way to handle sets of loose items in quantities of more than 50. Additionally, it appears that counting is rigidly associated with the decimal system, on the one hand, and learning written forms of numerals, on the other. Neither is strictly necessary or necessarily true.