Not sure why you think it's a lie, when it is not only the truth but hardly secret and is a long-standing practice.

DL's merchant acquirer, just like most, employs anti-fraud software which flags some transactions as sufficiently suspicious as warranting the presentation of the physical card. While there is no way to know exactly what triggered your situation (could have been entering the wrong CVV or expiration date or transposing digits in the card number. Could also be that you purchased a one-way ticket or are originating from a high-fraud location (which LED most certainly is). Not worth trying to figure out why and it could be any number of other reasons as well.

If your card is stolen, there are alternative means and you should contact DL immediately so as to not cause even lengthier delays at LED on departure.

To be precise, the merchant acquirer does not require it, but will not cover DL's loss if the card turns out to be fraudulently used. If consumers want anti-fraud protection through the chargeback process, it is fair for vendors to protect themselves.