1)
Why did the angel think that Zecharyah understands?
1.
Malbim: The matter itself - the Menorah and the seven Neros, these are known to those who came to Sod Hashem. Also the Menorah in the Mikdash hinted to these things.
2)
What do we learn from "Lo Adoni"?
1.
Malbim: Even though I know the Sod of the Menorah, I do not know what this vision hints to.
2.
This shows that [at least sometimes,] angels not know what people are thinking. Tosfos (Shabbos 12b DH she'Ein) said Stam that they know people's thoughts. (PF)
