You don't seem actually familiar with doomer talking points. The classic metaphor is that you might not be able to say how specifically Magnus Carlson will beat you at chess if you start the game with him down a pawn while nonetheless knowing he probably will. Predicting
The main way doomers think ASI might kill everyone is mostly via the medium of communicating with people and convincing them to do things, mostly seemingly harmless or sensible things.
It's also worth noting that doomers are not (normally) concerned about LLMs (at least, any in the pipeline), they're concerned about:
* the fact we don't know how to ensure any intelligence we construct actually shares our goals in a manner that will persist outside the training domain (this actually also applies to humans funnily enough, you can try instilling values into them with school or parenting but despite them sharing our mind design they still do unintended things...). And indeed, optimization processes (such as evolution) have produced optimization processes (such as human cultures) that don't share the original one's "goals" (hence the invention of contraception and almost every developed country having below replacement fertility).
* the fact that recent history has had the smartest creature (the humans) taking almost complete control of the biosphere with the less intelligent creatures living or dying on the whims of the smarter ones.