Their docs link to this explanation, in which the link I give uses an example that appears to be identical to the HA implementation - https://blog.cloudflare.com/validating-leaked-passwords-with...
I might be misunderstanding, but if the first 5 digits is a unique hash in the HIBP database then an attacker can replay that request, then lookup the hash. But "When hashes are grouped together by a Hash Prefix of 5 characters, we find the median number of hashes associated with a Hash Prefix is 305." {ibid}.
If you've acquired leaked passwords as HIBP have, you then just make hash lookup tables, then you have 305 potential passwords to try, or try and whittle down. If the bucket of hashes has "hunter2" in it, for example, then that's perhaps a good one to try - https://blog.cloudflare.com/validating-leaked-passwords-with... is worth reading.