But Christ ""was"" bound by the ceremonial law in order to fulfill it perfectly.
Under that law the sacrifice for sin and the sacrifice for thanksgiving were two distinct sacrifices.
The sacrifice for sin could not be mixed with leaven.
The sacrifice for thanksgiving had to include a leavened loaf.
This prevented them from being mixed together.
Furthermore, if sin were still being imputed against one under the Law,
and he had not yet given a sacrifice for his sins,
any sacrifice for thanksgiving was unacceptable.
Christ offered once and finished the sacrifice for sin on behalf of those whom His Father had given Him (John 6).
He proved He had finished the sacrifice for sin by rising from the dead.
The night before that sacrifice,
He showed the sacrifice which they were to offer from then on.
That sacrifice was the sacrifice for Thanksgiving.
"Eucharist" means "thanksgiving".
This is why He referred to the Supper elements as memorials of His death,
saying to do them "in remembrance" of Him.
A memorial is not an actual thing,
but a reminder of that thing which passed.
The first believers were Jews who knew the Law well.
They would never have confuted the two sacrifices.