The TOS aren’t referring to content-type headers, magic bytes, TCP headers, browser support of file formats, or any technical implementation.
To oversimplify, they’re saying Cloudflare’s service is to be used for serving websites to browsers.
Serving a static text file that is primarily used by applications is not in line with their terms of service.
Cloudflare provides a significant service to the free and open web by subsidizing the hosting costs of static content for websites. They give that away for free under what appears to be reasonable terms. I’m not sure why you’re trying to “gotcha” through their ToS.
It would be great if Cloudflare would donate resources to EasyList - it would do a lot to help the free and open internet by giving users more power over what gets delivered to their browser. But call that what it is: a donation.