how? They had a ton of races in the FRCS back in the day that aren’t core. This is evidence that Mercer has allowed people to pull from various D&D supplements to build characters and that they want the material in one place. It’s no different than Mordenkainen and Out of the Abyss both having Demon Lord stats.
Its not just Wildemount, its the broader trend, if a race keeps popping up in books, multiple settings its a sign that its a core race. Goblins were a core race for most MtG settings, so now that they are getting the D&D treatment it makes sense to add them to the PHB, especially since they are playable in Forgotten Realms, Eberron, Exandia, and other more traditional D&D settings. I'd also add Aasimar and Genasi to the PHB.