LLMs [...] no intelligence or capability to comprehend
That's debatable. LLMs have shown emergent behaviors aside from what was trained, and they seem to be capable of comprehending relationships between all sorts of tokens, including multi-modal ones.
Anyway, Stable diffusion is not an LLM, it's more of a "neural network hallucination machine" with some cool hallucinations, that sometimes happen to be really close to some or parts of the input data. It still needs to be "smart" enough to decompose the original data into enough and the right patterns, that it can reconstruct part of the original from the patterns alone.