Even though Aaron claimed that they were having a feast to the LORD, it was clearly blatant pagan idol worship that they learned from Egypt.
Therefore, this can't be considered as trying to worship the True God, as it is so far from true worship.
But what if you truly, sincerely want to worship the True God and unknowingly use pagan practices to do so, is that wrong?
Will God hold you accountable for something like that?
Let's see what the Bible says...