My 2006 C class estate has them as standard, I would be surprised if they were not already installed in a 2018 model C class.
Whatever you do , do not follow that idiot in the video. The fitting of all seat belts and restraints are what the industry call an 'A' grade process, which basically means 'Life or Death' process and are carried out by workers trained in that process using calibrated fitting equipment.
The least you need to do is find the EXACT torque setting for the bolts supplied with the MB kit , and stick to it.