In order for a collision to take place, we’d have to get a new connection from an existing client, AND that client would have to use the same port number that it used for the earlier connection, AND our server would have to assign the same port number to this connection as it did before.
Ephemeral ports aren't assigned to inbound connections, they're used for outbound connections. So, for the client-to-nginx connection, both the server IP and port are fixed (the port will be either 80 or 443) - only the client IP and port change, so for a collision all you need is for a client to re-use the same port on its side quickly.
For the nginx to node connection, both IPs and the server port are fixed, leaving only the ephemeral port used by nginx to vary. You don't have to worry about out-of-order packets here though, since the connection is loopback.
Note that only the side of the connection that initiates the close goes into TIME_WAIT - the other side goes into a much shorter LAST_ACK state.