The only glimpse I had of the inside of that rim suggested it had a pattern in it, probably to help it mechanically grip the tyre bead. Tubeless rims inside are smooth to get the best air seal. Also, tubeless wheels generally have an internal lip either side of the central well, designed (I think) to help prevent the tyre coming off the wheel in the event of a deflation. Once the bead of the tyre gets into the central well, it can come off the rim the other side. The lip is there to stop the bead getting into the well.
Geezer of my acquaintence sealed the spokes on his Africa Twin wheels with some clever tape.