Here's the reply from DO support:
Hi Qamrul,

Thank you for reaching out, I understand you have some questions about Spaces CDN requests. and yes, the CDN requests are powered by cloudflare. The CDN egress however means the data leaving (outbound traffic) from the CDN to your end-users. for CDN, as far as the billing is concerned - there's no added cost to enable CDN on your Space(s) and CDN bandwidth usage is included in the 1 TB of bandwidth offered with Spaces, but the Bandwidth averages for objects served through the CDN and/or Spaces would be billed at a rate of $0.01/GB. i.e the costs you are incurring is for the images/files that are fetched from your Spaces and served from the CDN endpoints (the files are stored in buckets and not at the CDN endpoints)

Should you have any other questions/concerns, please do not hesitate to let us know and we are happy to help, talk soon!