Iptables (and the successor nftables) is a powerful packet filtering system in the Linux kernel, able to create advanced firewall capabilities. One of the features that it cannot provide is per-application filtering. Together with SELinux however, it is possible to implement this on a per domain basis.
SELinux does not know applications, but it knows domains. If we ensure that each application runs in its own domain, then we can leverage the firewall capabilities with SELinux to only allow those domains access that we need.