Something like 1% of HTTP hits pose some risk of spam or fraud, those where somebody is trying to post a message or a transaction or something. The other 99% are just requesting a static HTML document or JPEG (or its moral equivalent with unwarranted and untrustworthy dynamic content injected). Static file serving is very difficult to DDoS, even without a caching CDN like Fastly. There is still a potentially large bandwidth cost, but generally the DoS actor has to pay more than the victim website, making it relatively unappealing.
Should web sites really be weighing "a thousand signals" to decide which version of the truth to present a given user with? That sounds dystopian to me.