My uneducated guess, fill the cavity with a reactive gas, then apply heat in one part of the process to react the gas with the walls, leaving a vacuum. Part of

My uneducated guess, fill the cavity with a reactive gas, then apply heat in one part of the process to react the gas with the walls, leaving a vacuum. Part of the article mentions silver cladding on the inside wall, which would be a getter for some gasses, with the downside that it is slightly tarnished. Or, the entire process is performed under vacuum. More likely, that part of the manufacturing process isn't fully solved, so they just have pumped samples.