The short answer to your question is Stephen was not wrong. The angel of the Lord that Stephen talks about is the same being that spoke to Moses in the Old Testament and guided Israel through the wilderness.
Acts 7:30, "And after forty years had passed, an angel, appeared to Him/Moses in the wilderness of Mount Sinai, in the flame of a burning thorn bush." In this verse notice the word "an" is used meaning, "an angel" appeared etc.
At Acts 7:38 it says, "This is the one who was in the congregation in the wilderness together with "the" angel who was speaking. Are you aware that the definition of the word "a/an" is different from the word "the."
The chief grammatical function of "an/a" is to connote a thing not previously noted or recognized. The word "the" connotes a thing previously noted or recognized. Even though at Acts 7:30 Stephen used the word, "an" he in the same verse identified "the" angel of the Lord appearing in the burning bush at Exodus 3:2.
Moreover, it was the angel of the Lord who was in the wilderness with Moses according to the Apostle Paul at 1 Corinthians 10:1-4. Cross reference that with Exodus 13:21, Exodus 14:;19 and Isaiah 63:9.
From Acts 7:30 Stephen explicitly mentions the events at Exodus 3 and who is the one doing the talking. Exodus 3:4, "When the Lord saw that he/Moses turned aside to look, GOD called to him from the midst of the bush, and said, "Moses, Moses," And he said, Here I am."
Notice Exodus 3:6, "He said also, "I am the God of your fathers, the God of Abraham, the God of Isaac, and the God of Jacob."
I have to bring the following up because it is "vitally" important that "the" angel of the Lord is "NOT" an actual angel, he is simply a messenger. This also means that "the" angel of the Lord is "NOT" Michael the arc angel. It's impossible for the angel of the Lord to be Michael or any other actual angel.
Here is why? At Genesis 22:11 the angel of the Lord calls out from heaven to Abraham. The angel of the Lord calls out from heaven a second time at Genesis 22:15. What does he say to Abraham?
Genesis 22:16, "By Myself I have sworn, declares the Lord, because you have done this thing, and not withheld you son, your only son." Verse 17, "indeed I will greatly multiply your seed as the stars of the heavens, and as the sand which is on the seashore."
The point is the fact that an angel or any angel cannot swear an oath on behalf of somebody else, let along for God. Hebrews 6:13,14, "For when GOD made the promise to Abraham, since He could not swear by no one greater, HE SWORE BY HIMSELF, verse 14, saying, I will surely bless you, and I will surely multiply you."
This could not be more clear. Sure angels and people/prophets represent God because the Jews have what's called the "Shaliah" principle. This means that a person/agent/emissary/envoy can represent a "principal" concerning the affairs of business etc. However, the shaliach remains a subordinate and he cannot swear oaths on behalf of the principal. One last point! "The" angel of the Lord never appears in the New Testament as the angel of the Lord.