I’m sure they appreciate the work but Apple has internal documentation on their chips. It’s not like they make a black box and forget how they designed them when it comes to writing software for them.
Some times an outsider's attempt to reverse engineer your product brings new insights.
That is true, but one does not simply implement a fully conformant bit shuffle instruction by accident.
Or sometimes an instruction isn't in the compiler because the hardware has some strange corner case failures so they aren't using it. I would be nervous about depending upon an unused instruction.
The article claims that Apple's compiler doesn't use the instruction, so it seems like the compiler author did indeed forget or never learn about it.
Or maybe it has some limitations they realized and blacklisted the instruction?