To my knowledge, yes, the final assembly (at least disks, RAM, battery, charger…) of the laptops is done in the US. I assume they receive the devices as a (custom made) barebone (case with PCB, CPU, display…).
Likewise, the Mini (and the server?) are customized barebones.
For the Librem key, and the Librem 5 USA, it’s not only the assebly, it’s also the production of the PCB.
As Purism grows, they’ll try to go more in that direction, having more control over the manufacturing process.