The whole chpt is about how Israel persecuted the prophets (Moses being one of them); and now they (at the time of Stephen) had persecuted and killed THE Prophet [raised up unto you of your brethren] sent unto them (Jesus; Acts 3:22,13,26--"His Servant Jesus") to speak unto them. ("whom ye slew" Acts 5:30)
The angel "appeared" but it is very obvious that it was the Lord who spake to Moses in mount Sinai:
Acts 7:30-39 And when forty years were fulfilled,
an angel appeared to him in the wilderness of Mount Sinai, in a flame of fire in a bush. (31) And when Moses saw it, he wondered at the sight: and as he drew near to behold,
there came a voice of the Lord, (32) I am the God of thy fathers, the God of Abraham, and of Isaac, and of Jacob. And Moses trembled, and durst not behold. (33) And
the Lord said unto him, Loose the shoes from thy feet: for the place whereon thou standest is holy ground. (34) I have surely seen the affliction of my people that is in Egypt, and have heard their groaning, and I am come down to deliver them: and now come, I will send thee into Egypt. (35) This Moses whom they refused, saying, Who made thee a ruler and a judge? him hath
God sent to be both a ruler and a deliverer
with the hand of the angel that appeared to him in the bush. (36) This man led them forth, having wrought wonders and signs in Egypt, and in the Red Sea, and in the wilderness forty years. (37) This is that Moses, who said unto the children of Israel,
A prophet shall God raise up unto you from among your brethren, like unto me. (38)
This is he, that was in the church in the wilderness with the angel,
that spake to him in the Mount Sinai, and with our fathers: who received living oracles to give unto us: (39) to whom our fathers would not be obedient, but thrust him from them, and turned back in their hearts unto Egypt,