>>>Screws should be made of a softer material than that into which the threads for said screw have been tapped. <<<

That not a rule I've hear of or seen applied. Screws have to be hard so the heads don't strip, much harder than the aluminum much equipment is made of. The only reason that a hole will strip is if the fastener was over torqued, the thread have corroded or galled, the joint was subjected to well over the intended load, or repeated disassembly/reassembly cycles have worn them out. If you do strip out threads, it's common to fix them using Helicoil inserts or similar. This generally produces a stronger thread than that in the native material. In some applications in engines and such, Helicoils are used from the outset to improve strength.