The question in the Torah is why certain ideas are given short shrift, while some concept are explored in great detail. The materials and building of the mishkan is given lots of pesukim while stuff like Tfillin and succah is given short shrift. The message is that giving or separating from oneself is the essence of being Jewish.
The Seforno appears to be saying that Moshe upon coming down from the mountain was about to tell the people about giving for the mishkan and about to tell the sanhedrin to order this commandment. However the Jewish people did not wait to be told by the sanhedrin. They gave “ad blie di”.
The message is simple, giving is the essence of Judaism. Giving of ones heart, compassion, and money. Jews don’t wait to be told to be compassionate.