I don't hold it as axiomatic. You can read arguments for this position in works like On Liberty by Mill https://www.econlib.org/library/Mill/mlLbty.html . But it seems obvious to me that it is true in that if there is a single predominant channel of information, any disinformation propagated on that channel will gain much more credence than if multiple competing channels are available. For example, you can read on how the war in Ukraine in its initial stages gained popular support in Russia as a mission of liberation and "de-Nazification." In support of this propaganda effort, independent news sources were shut down.