Yes, I'm making assumptions. However, the ones in your picture are almost surely low carbon steel vs. alloy steel in the case of the ones intended for frame builders. If you look closely at your picture, you can see the ends are just tack welded on. This application doesn't really require a lot of strength and the rack ends are mostly in compression. If you think of the millions of miles of abuse tourists gave the old Pletcher rat trap racks and the extremely low grade materials they were made of, strength really isn't a worry.
The ends on the rack in your picture probably extend fairly far up into the tubing. This makes up for a lot of sins. It would be difficult to replicate this design. Success really depends on your fabrication skills and equipment. The tabs companies sell to framebuilders are intended for use in a way that is tailored for construction techniques that are available to framebuilders but might not be that easy to use for someone starting from scratch.
When I make stainless racks, I cut my own tabs out of bar stock. You could make a tab that fits up in the tubing and then close the gaps on the sides of the tab with weld or even just crush the tubing. A hammer will do that pretty well if you are careful. It's something to consider.