If you take into account the retcons for Milotic, Vikavolt, Glaceon, and Leafeon, it's possible Pokemon can evolve through multiple means (even if they're not split evos) but the player is only ever given access to 1. So for instance Charjabug has always been available to evolve via both Thunder Stone and intense magnetic fields, but the SMUSUM protagonist never thought to try it on Charjabug, while there is no magnetic area in Galar so the SWSH protagonist has no choice but to try the stones.
So it's possible Phantump can also evolve if it lives for a very long time or if it comes into contact with an item we've never had access to.
