Yes, but not necessarily for any given client.
IPv6 is putting quite some emphasis on autonomous client configuration.
Your above description would match a client that uses Stateless DHCPv6 (as opposed to Stateful DHCPv6 where a client obtains a full lease much like via IPv4's DHCP).
However, clients may not use DHCPv6 at all, but instead employ SLAAC and learn DNS servers via router advertisements (RAs). Most modern OSs provide support for either way, but some OSs are restricted to certain modes only (e.g. Android does not support DHCPv6 at all).
You may enable Pi-hole's IPv6 support via Settings|DHCP (tick Enable IPv6 support (SLAAC + RA)) , but as long as your router is advertising itself as DNS server alongside Pi-hole, actual selection of DNS servers would happen entirely at a client's discretion.
If you cannot configure your router to not distribute an alternate IPv6 DNS server address (thus allowing clients to bypass Pi-hole), your only option may be to disable IPv6 altogether (if your router allows).