Typically this is the case, but WiFi is not actually defined as an interface to Ethernet (IEEE 802.3)
WiFi is a subset of IEEE 802, which deals with LAN/MAN variable-size packet networks.
WLAN (as defined by IEEE 802.11) can be used as an interface to multiple types of networks, or just as a standalone WLAN with no Ethernet backbone. e.g. a WLAN in promiscuous mode.