At the Last Supper the old covenant was still in effect.
Now, in Acts, Peter states that he had never eaten anything unclean.
As an observant Jew, Peter would not have eaten something that he thought was unclean, so if he thought Jesus was telling them they were eating His literal flesh and blood, Peter would have refused to eat. PETER understood it to be symbolic.
Now, if he had eaten blood at the last Supper, he would have eaten something unclean, and therefore lied to God about never doing it.
Nice catch! Exactly!