Yes, it is, well, tld is not accurate, Let's Encrypt uses the publicsuffix-go and this list gets the data from Public Suffix List to know what are the tld domains... but this list also includes non tld domains. In your case, cu.cc is not included in that list so your domain.cu.cc is not a primary domain for Let's Encrypt but a subdomain so, if lot of people uses a cu.cc domain to issue a cert, the 20 certs per domain limit is reached quickly and could be almost impossible to issue a cert for your domain.
cu.cc owner could request to be included on this list but not only to allow issue certs for the subdomains but also to avoid privacy-damaging "supercookies" being set for high-level domain.