No, a firewall is an application, a process that brokers all incoming and outgoing communications and maintains a state table of those inbound and outbound connections. The key there is that it maintains a state table. TCPd is a shim process that acts between inetd and the actual application. It is not a firewall. It doesn't drop packets. It doesn't maintain a state table, so it can't, for instance, handle reflexive policies or tell whether or not a dialog has been established. It does handle access control for applications based on IP. However, there's a difference between a firewall saying "you aren't on my allowed hosts list, DROP" and inetd saying "packets accepted, looks like you want to launch application X, tcpd, is that cool? No? ok, sorry, not allowed. SIGTERM." In the end you get similar results, but they're significantly different processes.
This is why I strongly disagree with the idea that firewalls are always needed. They're just another tool, and there are other tools that do similar things.