Soundsmith said that the aluminum they use is not the same as the aluminum that the original cantilever was made from
Exactly, those aluminum cantilevers on Ikeda and Takeda cartridges are special and this is one of the reason those FR-7fz and Miyabi are so good (don't mix it up with conventional aluminum cantilevers anyone can buy today). Also aluminum is easier to work with for cartridge designers according to Mr. Andreoli. In Japan aluminum still a good choice for cartridges like Miyajima Kansui and its designer said Boron doesn't improve anything (at least for him).
I assume that re-tippers simply can't get equal aluminum cantilevers anymore (for a long time) and they can get only what is available (Boron Rob, Ruby, Sapphile or dirt cheap aluminum).
What is NEW from those jewel companies is Zirconia Pipe cantilever.
Well, at least something new!