The reason you don't see spam in Kagi is because nobody is targeting Kagi specifically.

They can probably get away with a lot of stupid rules that would backfire if anybody tried to cater to them specifically.

"SEO spammers being more advanced than multi-billion-dollar search conglomerate" is a myth. Spam sites have an obvious objective: display ads, shill affiliate links or sell products. All these have to be visible, since an ad or product you can't see/buy is worthless. It is trivial to train a classifier to detect these.

But let's play devil's advocate and say you are right and spammers are successfully outsmarting Google - well, Kagi does use Google results via SerpAPI by their own admission, meaning they too should have those spam results. Yet they somehow manage to filter them out with a fraction of the resources available to Google itself with no negative impact on search quality.