These last seemingly repetitious words “so they did” actually mean “Precisely they did.” Ken (so) is sometimes used in the sense of “precise” or “correct” (as “Correctly do the daughters of Zlafchad speak”—Bamidbar 27:6, where the word Ken is used for “correctly”).
The sons of Israel were tested, after receiving the Torah (and after the experience of the Golden Calf), where they had learned the lesson of perfect obedience to Hashem’s word through Moshe, without recourse to their own reasoning.
The work of the Mishkan included thousands of details, almost all of which had no given reason; and by the many thousands of persons that participated, very many more desirable designs or arrangements could have been substituted.
What difference could it make? Yet all of them had already learned the Torah lesson:
Hashem’s words by means of Moshe were to be fulfilled with the utmost precision, faithful even in the most hair-splitting detail. “So they did.”
This great generation of original minds learned that Hashem’s word is of universal importance, even in its most minute detail.
Their children after them, as is everywhere evident in the Mishnah and Talmud and later books, continued in this precise unswerving loyalty to every minute detail.
The nations look on in amazement how the Sages battled over seemingly unimportant and rarely-occurring details, and pages are occupied with these discussions.
But Israel learned that the infinite greatness of Hashem bestowed an infinite greatness on the smallest portion of His commandments; for infinity cannot be divided into finite parts. (A Nation is Born)