Using HTTP POST for denial of service

same thing, but generally CDNs are used for static content (lots of GET requests). some support POST for uploading large files (images, video, etc), but that's less common.

you don't generally put a CDN in front of your 'dynamic' web server domain (because for dynamic content it can't usually help with caching, and just adds another layer of indirection and delay)

layer 7 load balancing proxies will almost certainly 'fix' slowloris AND this 'slowpost' attack - that's what nginx or any of the other servers I listed are doing - DNS round robin etc (poor man's balancing) obviously won't help.