She is correct.
In the Democratic primary process, there is no such thing as a pledged delegate.
The rules do not require the delegates to vote for any particular candidate, even on the first ballot.
So ANY delegate can vote for anybody they want. It is their right.
However, the CANDIDATES have the right to replace any delegate they have won. So practically speaking, you wouldn’t expect many delegates to switch. They are hand-chosen by the candidates based on their loyalty, and if they show any signs of wavering, they can be replaced by the candidate.
We're talking Democrats here. If they decide to sell out to the other side, they would never let the candidate who chose them know, before it's too late for them to do anything about it.
So why do they even bother letting the public vote if delegates can do anything they want?