All I know, when I was issued a subpoena by a civil court I complied.
This is a subpoena issued by a court in a criminal case. Much more serious.
I think Flynn's still holding out for immunity, thus Trump's admonition to stay strong in refusing to play along.
That may well work in civil court, where people's lawyers can play games until the other side drops it.
Trump and his minions are dealing with Federal criminal court, way different circumstances.