The seed node would not work without the p2p plugin, but the plugin will be loaded automatically, because it's a requirement for the network_broadcast_api.

From the 'appbase/README.md':

All plugins complete the Initialize step before any plugin enters the Startup step. Any dependent plugin specified
by APPBASE_PLUGIN_REQUIRES will be Initialized or Started prior to the plugin being Initialized or Started.

network_broadcast_api_plugin.hpp:

APPBASE_PLUGIN_REQUIRES(
   (steem::plugins::json_rpc::json_rpc_plugin)
   (steem::plugins::rc::rc_plugin)
   (steem::plugins::chain::chain_plugin)
   (steem::plugins::p2p::p2p_plugin)
)