"I know that the LORD will maintain the cause of the afflicted" (140:12). The psalm never says the psalmist was rescued. How can he say "I know"?

The psalm doesn't say what happened after it was prayed, and it doesn't explain the words. What it gives is what the psalmist says before them. He calls the LORD his own: "I said to the LORD, “You are my God.”" (140:6). He speaks of what the LORD has already done: "you have covered my head in the day of battle" (140:7). He doesn't say which day of battle that was, or whether the men of 140:1-5 were still around him. After "I know" (140:12) comes one more verse, and it too says what will be: "Surely the righteous will give thanks to your name. The upright will dwell in your presence" (140:13). These notes don't add an ending the psalm doesn't have.