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.)