Storage snapshots for consistent backup of postgresql - different data and log volumes
The documentation you cited says it all, but I wouldn't blame you if you want to try to verify the claims of the vendor regarding snapshots taken at the same time. Perhaps a way of uncovering something could be to stress test the WAL system more specifically.  

e.g. In addition to your pgbench-based tests, try adding random calls to pg_switch_xlog() to force log rotation, shorter and longer checkpoint intervals (shortening and lengthening checkpoint_timeout and checkpoint_timeout) and even using small or large wal file sizes. 

Unless there's something I'm missing, for your snapshots not taken at the same time, I would attribute your recovered DBs perhaps to a bit of lucky timing. In the last case, imagine you took your log snapshot while the current xlog location was, say, 0/A1C0FFEE. Then you have 3 minutes of particularly heavy load on the system, that causes a full cycle through the WAL files, and your DB is now at 0/DEADBEEF when the data snapshot is taken. When you attempt to restore, the WAL files being written to at the time of the data snapshot are long gone, and recovery will fail.