Theophylact of Ohrid

Then the disciples understood that He spake unto them of John the Baptist. Then the disciples became keener in perception and understood that He was calling John Elijah, as John was the Forerunner of the first coming, just as Elijah would be the forerunner of the second coming.

Commentary on Matthew ↗

Jerome

And so the Son of Man will suffer at their hands. Then the disciples understood that he was speaking to them about John the Baptist. Now, as Herod and Herodias are said to have killed John, how can they also say that they crucified Jesus, when we read that he was killed by the scribes and Pharisees? The answer is that both the Pharisees were in agreement with the death of John, and Herod willingly participated in the killing of the Lord. He sent him, mocked and despised, to Pilate to be crucified.

Commentary on Matthew ↗

Thomas Aquinas

Then the effect of this response is presented: then the disciples understood that he had spoken to them of John the Baptist. Then: when the Lord had spoken to them. The declaration of your words gives light and gives understanding to little ones, Psalm 118:130.

Commentary on Matthew ↗

Rabanus Maurus

From the mention of His own passion which the Lord had often foretold to them, and from that of His forerunner, which they beheld already accomplished, the disciples perceived that John was set forth to them under the name of Elias; whence it follows; Then understood the disciples that he spake to them of John the Baptist.

Catena Aurea by Aquinas ↗