Wiped everything with TWRP, re-flashed CM13 and filled the internal storage with dummy data. Is my data now gone? I sold my OnePlus One and needed to remove files from the internal storage and re-install CM13. I deleted all photos and videos by hand in Windows. Then I selected all the options available inside TWRP and wiped it. After, I installed CM13 from an image. Then I filled the phone in Windows with dummy data by copying. My phone was not encrypted before wiping. 

Can I consider my data more or less gone? I'd imagine at least most of the photos and videos are gone with overwriting but what about application data such as SMS, Facebook etc. messages? If they're still on the phone, how hard would it be to bring them back to life?
You must have performed a clean wipe before you flashed the rom CM13 . At this point the data related to your apps are already gone. @acejavelin made a correct point. Recovering that data is near 0.