It's actually not. The "sufficient complexity" part is doing all the heavy lifting. If a proposed physical system is not conscious, well clearly it's not sufficiently complex.
Am I saying LLMs are sufficiently complex? No.
But can they be in principle? I mean, maybe? We'll have to see right?
Saying something is possible is definitely an easier epistemic position to hold than saying something is impossible, I'll tell you that.
For all consciousnesses that exist, they arose from a (complex) physical system.
What you are positing is, effectively:
Any physical system (of sufficient complexity) could potentially become conscious.
This is a classic logical error. It's basically the same error as "Socrates is a man. All men are mortal. Therefore, all men are Socrates."