How does this article support the idea that WeChat raises concerns about monitorig by a surveillance state?

The Chinese authorities in the article didn't "monitor" anything. They retrieved the deleted message from the phone itself. For all we know, it could be as innocuous as running a SQLite DELETE to delete the message and the scheduled vacuum hasn't run yet. I don't see any convincing evidence of either actual monitoring or Tencent conspiring to allow such monitoring.