Implicit type GetComponent call

I read the following article on MonoBehaviours and abstraction (very interesting BTW). I have difficulties to understand a part of his article, where he seems to implicitly type his generic GetComponent calls. At some point he uses:

 GameObject projectile = 
   (GameObject) GameObject.Instantiate(projectilePrefab, owner.transform.position, Quaternion.identity);
 projectile.GetComponent().SetMovement(new LinearMovement());

Later he defines the following extension methods

 using System.Collections.Generic;
 using System.Linq;
 using UnityEngine;
 
 internal static class ExtensionMethods
 {
     public static T GetAbstract(this GameObject inObj) where T : class
     {
         return inObj.GetComponents().OfType().FirstOrDefault();
     }
 
     public static T GetInterface(this GameObject inObj) where T : class
     {
         if (!typeof(T).IsInterface)
         {
             Debug.LogError(typeof(T).ToString() + ": is not an actual interface!");
             return null;
         }
         return inObj.GetComponents().OfType().FirstOrDefault();
     }
 
     public static IEnumerable GetInterfaces(this GameObject inObj) where T : class
     {
         if (!typeof(T).IsInterface)
         {
             Debug.LogError(typeof(T).ToString() + ": is not an actual interface!");
             return Enumerable.Empty();
         }
         return inObj.GetComponents().OfType();
     }
 }

he then calls these methods this way:

 GetInterface().Open();

I don't understand how this is possible, I've never seen any GetComponents call without specifying the type between . He seems to implicitly define the type "Any type with a specific method", but when I try something similar it does not compile. I get the following error:

 Constraints are not allowed on non-generic declarations

Is there a mistake in the article or am I not understanding what is going on?