?

Even Steam Deck has a custom APU made specifically for it. Call it "customized" if you don't like "custom".

The steam decks APU was designed for Magic Leap 2. It is one of the worst examples you could choose for a chip specifically designed for the thing it is in.

No, they mean customized SoCs which what those chips are as well

So "run on a custom silicon" should not mean anything "custom" in regards to architecture or ISA or even microarchitecture?

If that is true then a lot (or most) of android phones run on "custom silicon" and the term is meaningless.