In theory it's fine to have multiple DHCP servers as long as they don't step on each other. But in reality a bunch of the things mentioned in this post are ways for the network to be hosed. Unfortunately, enough LANs are semi-hosed that it's worth working around known problems.

That works mostly, you'll have problems if you have duplicate IP addresses though.

Windows supports dhcp failover, where multiple dhcp servers can exist on the same subnet.

Yeah… I feel like “falsehoods X believe about Y” need to omit anything a reasonable X would consider to be a bug.

VPNs

Port- or Client-isolation on Ethernet switches and Wi-Fi access points respectively. Often seen in corporate environments and (properly configured) guest networks.

I feel like the better of this is maybe "will" instead of "can"