Also, doesn't Cloudflare require a specific pair of DNS name servers in order to assert that someone "owns" a domain?
I had also created my "worker pages" app/project in cloudflare before updating my nameservers in spaceship.
But i do have namecheap domains as well and used firebase hosting (google) and they DID ask to have certain TXT records to be added to namecheap and validated before provisioning my ssl cert.
I'm assuming every provider is different. But yea i would just undo the CF nameserver until you're ready to go live
It would be easy enough to automate the creation of accounts until you have an account for every nameserver pair, then start serving on unclaimed domains.
I learned this when I was transferring a whole bunch of domains over, and I noticed it kept asking me to use the same nameservers. So I started setting the nameservers in advance of trying to import the domain into Cloudflare, to save a little time. And then... Cloudflare started asking me to use different nameservers, and wouldn't serve content until I did.
So even if you create 2500 accounts trying to claim the same domain, I suspect it will never give you the pair that's already configured, and therefore you won't be able to claim it. I also suspect it won't give any two accounts the same nameserver pair.
(I'm a Cloudflare employee but my knowledge above is only based on my observations as a user; I don't work on this part of the system. I have no idea what actually happened to OP.)