I wonder how much it costs them to maintain this service.
To be fair dummyimage.com doesn't do any sort of image reading or resizing. Source doe at https://github.com/kingkool68/dummyimage
In general I like to see brand signals: if someone I respect mentions that they use Cloudflare, that is useful information, even without further details.
Unsolicited mentions are usually useful, it's just sneaky paid advertising is bad.
Not possible to detect under these circumstances, then.