The article leaves lot’s of questions. The 12 apostles and Paul are not the only ones called apostles. Barnabas comes to mind.
It appears to me, that the early church considered there to be three kinds of apostles.
1. Christ Jesus, Heb. 3:1. Definitely a kind of its own.
2. The twelve
3. Paul and Barnabas, Acts 14:14
Neither Paul or Barnabas were of the twelve, yet they are called apostles...which would indicate the early church must have seen the original twelve as a special class, they being personally taught by Christ during his time on earth, then being sent by him.
While others, such as Paul and Barnabas, not taught personally by Christ during his time on on earth, yet were apostles, they being "sent" by the Spirit and by laying on of hands in the church at Antioch:
Acts 13 (KJV) 1 Now there were in the church that was at Antioch certain prophets and teachers; as Barnabas, and Simeon that was called Niger, and Lucius of Cyrene, and Manaen, which had been brought up with Herod the tetrarch, and Saul.
2 As they ministered to the Lord, and fasted, the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them.
3 And when they had fasted and prayed, and laid their hands on them, they sent them away.
4 So they, being sent forth by the Holy Ghost...
It, thus, seems to me, the early church considered being "sent," whether by the Father, as Jesus was, or by Jesus, as the twelve were, or by the Holy Ghost, as Paul and Barnabas were, the primary determinant as to the meaning of "apostle."
Again, it seems to me, that if the Holy Ghost could send Paul and Barnabas, with a Spirit filled church having lain hands on them, as at Antioch, this opens the door for others of this kind of apostle.
Of course, this leads into the bigger cessationist issue. Since most FReepers on this thread seem to believe the Spirit no longer can operate like it did in the early church (as at Antioch), then, of course, no apostles of the Paul and Barnabas class.