Why was Iblis's refusal to prostrate to Adam considered a sin, even though the command to prostrate was directed at the angels, and Iblis is from the jinn?

The correct view is that Iblis was not one of the angels; rather, he was from the Jinn. This is supported by Allah Almighty's saying: "And [mention] when We said to the angels, "Prostrate to Adam," and they prostrated, except for Iblis. He was of the Jinn and disobeyed the command of his Lord." (Al-Kahf:50). This view has been supported by Al-Hasan Al-Basri and Sheikh Al-Shinqiti.

His being from the Jinn does not preclude the command to prostrate to Adam. Indeed, Allah Almighty commanded the angels and Iblis to prostrate to Adam, and Iblis was included in the address to the angels and resembled them, so he was condemned for disobeying the command.