Is Transaction Log Filling up Because Log Truncation is Delayed?
Am I interpreting these results correctly?


Yes, you are. Although it's a system database, it doesn't automatically back itself up. You must add these to your backup schedules just like the user databases. 


  Can anyone recommend a way to free up space in the transaction log?


As you read, and understood, back it up. Once you back up the TLOG (which will require a FULL BACKUP first if you haven't done that) the % used will drop a lot. Then you can shrink the file. Go ahead and schedule these, or set the DB to SIMPLE MODE if you're good with that.

I should mention none of your log files are that big. They also probably aren’t set to a max growth so they probably won’t fill up until your volume does. Be sure to check them.