you can see on the non-drive-side rack attachment that the ends of the tubes are just crimped. It's unlikely that they are sealed in any way. Probably done in a hydraulic press.

here are some rack tabs, I usually just make mine since I want them to be stainless https://framebuildersupply.com/produ...and-3-8-tubing
Also these MS2007 - steel Rack lug 5 mm hole
LLewellyn makes some nice socketed rack lugs
Llewellyn Rack Boss Kit-1 set (4 pcs) Articulated bosses SS :: Llewellyn Frame Parts :: Nova Cycles Supply Inc.