Public DNS servers do not know anything about your local network, so this information has to be sourced from within your network originally.
"these requests" refer to local hostname lookups (A/AAAA) or reverse lookups (PTR) that will not produce a name or an IP respectively if Pi-hole has no way of determining them (so, indirectly to "...won't be able to determine...").
When Pi-hole is acting as DHCP server, clients requesting an IPv4 lease will also provide a hostname, and Pi-hole's embedded dnsmasq will create the appropriate DNS records, Those records will then be considered whenever a client requests local (reverse) lookups.
Pi-hole itself will routinely check reverse lookups for known local IPs.
If Pi-hole isn't your DHCP server, your router as DHCP server may (or may not!) create DNS records upon DHCP lease negotiation in its own DNS server. Your router may also allow to label a client with additional hostnames. Pi-hole then can divert local queries to your router, which will provide an answer (if known). This is what Conditional Forwarding does.
Alternatively, you could use your router as Pi-hole's only upstream DNS server. This would also give you local hostname resolution, but subjects control and choice of public DNS server to your router's limits.