for I know that through your prayers and the help of the Spirit of Jesus Christ this will turn out for my deliverance, (Philippians 1:19, ESV)Verse 18 ended with, "Yes, and I will rejoice..." which should actually go with this verse. So, he is rejoicing that the gospel of Jesus Christ is being proclaimed, but now he is going into another reason why he is rejoicing and continuing to rejoice.
Being in prison, it is not unusual at all that he knows they will be and have been praying for him. Even being confident in deliverance from this situation is absolutely fitting to the thought process. What will be interesting is what he considers to be his deliverance, which we will see in the next few verses.