Why does this signalr interface method not need a proper implementation?

Can't wrap my head around this piece of code of an example.

There is an interface that looks like this:

using System.Threading.Tasks;

namespace NotificationHub
{
    public interface INotifyHubClient
    {
        Task NotifyClient(string type);
    }
}

And a class that uses the interface:

using Microsoft.AspNetCore.SignalR;

namespace NotificationHub
{
    public class NotificationHub : Hub<INotifyHubClient>
    {
    }
}

In a controller it is simply used like this:

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.SignalR;
using System;

namespace NotificationHub.Controllers
{
    [Route("api/[controller]")]
    public class NotificationController : Controller
    {
        private IHubContext<NotificationHub, INotifyHubClient> _hubContext;

        public NotificationController(IHubContext<NotificationHub, INotifyHubClient> hubContext)
        {
            _hubContext = hubContext;
        }

        [HttpPost]
        public string Post([FromBody]string message)
        {
            _hubContext.Clients.All.NotifyClient(message);
            return "Success";
        }
    }
}

Why does this work without a proper implementation for NotifyClient?