He quotes Augustine earlier in the article:

In his City of God, completed by 426 AD, early Church father Augustine of Hippo writes of the general confusion surrounding this passage and its harrowing imagery of the Antichrist.

“Then as for the words, ‘And now ye know what withholds,’ i.e., ye know what hindrance or cause of delay there is, ‘that he might be revealed in his own time;’ they show that he was unwilling to make an explicit statement, because he said that they knew. And thus, we who have not their knowledge wish and are not able even with pains to understand what the apostle referred to, especially as his meaning is made still more obscure by what he adds. For what does he mean by ‘For the mystery of iniquity doth already work: only he who now holds, let him hold until he be taken out of the way: and then shall the wicked be revealed?’ I frankly confess I do not know what he means.” (City of God, Book XX, Ch XIX.)

Yes, even Augustine says, “I just don’t know.”