By default it implements rough limits that you cannot exceed to make sure that you do not not transmit too much noise. Additionally, you can always establish private channels for your nodes and / or not retransmit at all.

Meshtastic isn’t intended for mission-critical uses or as an internet substitute. It is intended for very basic text based communication (e.g. between your friends) or remote IoT devices.

The congestion argument also applies to all radio based communication, there are always people transmitting with high gain, noisy outputs or spam.