I'm not sure what exact kind of info CDP really provides, like if it contains a list of enabled VLANs in the link, but ESXi distributed vSwitches do in fact handle CDP/LLDP in both directions and can extract some information out of it. This is not implemented in the network health check function but separately from that.
You can see that in the esxcfg-info of my example above and in the network configuration section on the respective vmnic:
The vim-cmd networkhints command is just a misunderstood stepchild of the "observed IP ranges" which has been around since ages in that regard.