Looks like you're right. According to Wikipedia, part of the electrical interface for mini PCIe:
SIM card for GSM and WCDMA applications (UIM signals on spec.).
And reading the technical docs for my wireless card (a Sierra Wireless AirPrime MC73xx),
The module supports one SIM (Subscriber Identity Module) (1.8 V or 3 V)...The SIM pins provide the connections necessary to interface to a SIM socket located on the host device. Voltage levels over this interface comply with 3GPP standards.
In case anyone's curious, here's the MiniPCIe/SIM pinout:
||SIM contact number
||Power supply for SIM
||Bi-directional SIM data line
||Serial clock for SIM data
||Active low SIM reset
||USIM_GND is common to module ground
TL;DR: Motherboards supporting LTE cards wire SIM card pins directly to the LTE card over Mini PCIe reserved for a SIM card connection.
In order to use these cards, your motherboard needs a SIM connector, or you need to wire it up some other way. This probably explains LTE cards I saw for sale that said "For Dell only;" they must have used non-standard pins.