differential amplifier will cancel DC present at input
A differential amplifier will amplify a differential DC signal just as well as it amplifies a differential AC signal.

Similarly, it will have very little, if any, gain for common-mode signals, whether they are DC or AC.

If your source was somehow implying that DC signals are always common-mode, and that AC signals are differential, then the statement is sort of true, but it's conflating two completely separate issues, which makes it confusing.