"Whom they rejected": that is, they rejected him as ruler and judge. They did not acknowledge him as such, although by virtue of the later divine election he was such.
Stephen generalizes the fact that one of the fighting Hebrews rudely pushed Moses away with reproachful words. He applies it to the whole people, in the sense that the words of one man expressed the untrustworthy and stiff-necked disposition of the whole people.
This man, rejected by the people, God sent as ruler and deliverer. The speech is plainly intensified. The Jews rejected him as ruler and judge. God sent him as ruler, and not only as judge, but as deliverer.
This intensification rests on the fact that Stephen here presents Moses as a type of the Messiah, the Redeemer of mankind. In the people's resistance to Moses he sees an image of the people's resistance to Jesus the Messiah.
As the people then rejected Moses, so now the people rejected Christ. Yet as God sent Moses as ruler and deliverer of the people, so He made Christ the Ruler and Deliverer of mankind.
The outward deliverance of the people by Moses from Egyptian slavery was an image of the spiritual liberation of all mankind by Christ from sin (cf. Lk 1:68; Lk 2:38; Heb 9:12; Titus 2:14). Thus Moses is a prototype of Christ.
"Through the Angel": literally, "by the hand of the Angel," that is, by the power of the Angel, who strengthens, protects, and helps. This is the power of the Son of God.