Transaction Log Truncating is replicated through DAG. So if you truncate on the passive Database, it is replicated to the other nodes. This is a core functionality from Exchange DAG (not from Veeam).
Here you can find a lot of informations:http://andyandthevms.com/exchange-dag-v ... plication/
Just an additional small comment for all the others.
Lately some of our customers contacted me and said that round about VMware SnapShot commit they face DAG cluster failovers.
When I analysed it, the reason was NFS Datastores are used with HotAdd backup mode. This is ugly in 2 ways:
1) NFS is not supported with Exchange (and there is a technical reason behind that) => see my blog above.
2) HotAdd with NFS can cause some VM stunns at DiskHotAdd release => See my Blog and VMware KB.
=> Change Datastore for Exchange to a block media or if you want to keep it really on NFS, use NBD-Network mode or the new v9 DirectNFS backup mode.