This is nothing new. Worked great for structural space frame race cars back in the '50s and '60s. It's a specialty alloy rod with fairly high tensile strength from what I remember (not just generic silicon bronze brazing rod), with a gasflux process (another specialized process). I think TIG overtook it, when TIG became commercially available, but it still has some advantages that I can't recall. The only time I've heard people talk about it was for race cars, and that the tensile strength is not too much different than an as-welded TIG bead that's not post treated at all, depending on the steel alloy. Obviously if you're doing post weld heat treatment of TIG welded structures, that would be superior.
I don't really know if there's any difference in joint design, but I would imagine there may be.
That's all I've got. So not exotic at all, just a process that really isn't used much anymore.
The only reason it was sifbronze brazed is because it was built in the UK and they still use those processes, not because it's any better, especially being regular carbon steel. You can clearly see a large HAZ on the tubing and lots of cold joints with the spacing. So maybe not the wrong process but maybe the wrong welder.