I can't read anything that would allow that conclusion from the source you've linked, but those reports would be incorrect anyway.
Conditional Forwarding is beneficial if your router runs a DNS resolver that knows the names of the router's DHCP client. In such a scenario, Pi-hole can query the router for such local names when CF is enabled.
An alternative would be to configure Pi-hole to use your router as its only Upstream DNS Server, as your router's DNS resolver would then see every DNS query that Pi-hole forwards, being able to answer the local names while forwarding the unknown names to its own upstreams (WAN).
This would be the only constellation where you could forego CF and still query the router's DNS for local names.
Now, if your router would use Pi-hole as upstream, that would close a.) a partial loop with CF enabled and b.) a full with Pi-hole using your router as upstream.
For b.), you'd need to point your router's upstreams to some public DNS resolvers, or you won't have DNS resolution at all.
Since your router distributes Pi-hole as local DNS resolver via DHCP, your DHCP clients DNS requests are filtered by Pi-hole.
For a.), you also point your router's upstreams to some public DNS, or you could consider to short-circuit the partial loop via Pi-hole's client-specific filtering, see e.g. Dnsmasq[1035]: Maximum number of concurrent DNS queries reached (max: 150) - #2 by Bucking_Horn
Note that you'd had to adopt the rules from the linked post for your network's IP range and search domain.