Telstra don't own the NBN, as such Telstra can't control how much backhaul support the nodes are provided over the NBN. The NBN network and its congestion issues are managed by NBN Co.
The issue isn't that Telstra are selling more then they can provide, it is that NBN Co have more connections, through all providers then they are providing the backhaul support for....
It wouldn't matter what provider you were with when it is NBN Congestion as the providers have no say over it, and can only report the issue to NBN Co in the hope that they will review it and upgrade the Nodes appropriately.
The issue isn't that Telstra are selling more then they can provide, it is that NBN Co have more connections, through all providers then they are providing the backhaul support for....
It wouldn't matter what provider you were with when it is NBN Congestion as the providers have no say over it, and can only report the issue to NBN Co in the hope that they will review it and upgrade the Nodes appropriately.