That is the flint on the SDW for you. As has been said patching will keep this tube going for miles to come yet. I have several similar slashes fixed on mine and run them tubeless no worries.
The tube/air will try and make that weak spot larger so yo will need to reinforce it. I have done this successfully with tube patches or chunks of old inner tube and normal tube patching glue. Clean the inside of the tyre in the same way as cleaning the tube and just patch it. If using a piece cut out of an old tube make sure it is clean and rough the surface a little. Apply the vulcanising solution to both and leave to go tacky. Then firmly press into place. Don’t forget to cover the patch with talc etc to stop any excess glue sticking to the inner tube.
Milk carton/toothpaste tube are great for on the trail but may move over time. Also make sure you have no corners.