They absolutely do, unless you believe they are lying about the fact they're using current generation models extensively to develop the next generation of their models.

The letter S stands for "Self" and word "using" is for sure not a superset of the word "self". Basically LLMs are assisting someone who does improving of said LLM, while RSI is a carpal... ahem, RSI is "self" improvement, meaning no intemediary in a human form. PS: it's also not recursive but iterative improvement, even if it ever happens.