That's false in the sense that they cannot be used to purchase anything directly from WDW, or book a stay at WDW completely separate from any cruise.
True in the sense that you can use your FCC to book, via the cruise line, a pre-cruise and/or post-cruise hotel and tickets at disney world. This must be booked via the cruise line as part of a package with an actual cruise (i.e., if your cruise is cancelled, so would your hotel stay).
WDW and DCL are separate companies, but DCL has an agreement with disney parks that gives it limited rights to book a cruise package that includes WDW room and ticket packages (however, inventory may be limited, and you would not be able to take advantage of any discounts offered by WDW to the general public).
This is similar to how you can use FCC to book a flight through DCL in connection with a cruise, but can't use it to book a standalone flight without any cruise.