From what I understand the S cars run like 10:1 rich or something and N/A engines make best power between 12.5:1 and 13:1 AFR... danger only usually exists when approaching 14:1 under heavy load, though richer is usually safer. Situations can arise where due to intake air temp or coolant temp the optimum AFR for best power becomes dangerous with the stock timing map, but the resultant damage is usually the piston, not the exhaust valve

9.x:1 air fuel ratio? Wow, that is pig rich lol...

or or were you referencing the static compression ratio?

I think most of the S cars were 9.5:1, though with some creative swapping of parts you can arrive at something like 10.2:1 and still technically be “stock”...
I think the combo is late rotating assembly with early flat faced valves that gives high compression, but can’t remember 100% since I haven’t had an S car in forever, and if I got one as a project I’d likely be wanting to Ecotec swap it...