Is it a valid excuse?

These were Moses’ excuses to God’s calling (Exod. 3:9-10):

"…  for I am slow of speech and slow of tongue" (4:10);

"...  for I am unskilled in speech" (6:12);

"...  I am unskilled in speech ..." (v. 30)

However, this is what the Bible Acts 7:21-22 says about Moses:

“And after he had been set outside, Pharaoh's daughter took him away and nurtured him as her own son.

"Moses was educated in all the learning of the Egyptians, and he was a man of power in words and deeds.”  

This was God’s promises to Moses:

“Now then go, and I, even I, will be with your mouth, and teach you what you are to say.” (Exod. 4:12)