There is planar travel, it is very feasible that a character from Eberron or Dragonlance, or even Darksun could find their way to Toril. The Doors of Sigil stretch to all planes of existance, and since the illithid did have a planes traversing nautiloid, races from everywhere could have been taken.
You don't even need planar travel to have Warforged being active on Toril. There are several Toril-based groups who could create them (artificers, Lantan and Red Wizards).
