The screws were metal, but they went into a plastic casing. Now, the metal screws go into metal threading

They are mostly (all?) metal.

Torx head are preferred for smaller screws because it is a lot easier to strip the heads with a phillips. Mostly because you can use too big or too small of a phillips head with a screw which means you don't have a good fit and are going to mangle it. Whereas a torx is very much "one size fits one size".

But also? If you actually pay attention to the video/read the article and are not a complete monster, you will use a ph0 instead of a ph1 or whatever and that stops being an issue. But it makes people happier and maps better to the ridiculously expensive electronics screwdrivers (cough, lmg, cough) that come with a very narrow set of bits rather than assuming people shell out almost the exact same amount of money for an ifixit kit that has dozens of bits. Or, you know, people who realize their local hardware store also sells bits.

The real advantage is that it sounds like Valve are moving away from self tapping screws. Explaining those is well beyond my brain, but it boils down to the idea that they cut/grind/clomp through plastic to hold themselves in place. That is why Valve have made it very clear that disassembling and reassembling your Steam Deck will lower thermal performance and durability. The screws won't go in as tight as they used to and, if you do it enough, they won't hold at all. If you ever were reassembling something and the screw just kind of spun freely, that is likely the cause.

When you buy "wood screws" that doesn't mean that the screws are made of wood, it means they go into wood.