Creatures that say this only resists B-P-S damage types
Damage resistances bludgeoning, piercing, notch of non-magical attacks
When a notebook indicates it, it means that only the creature is resistant to blunt, piercing, and non-magical attack reducing types. Since strength damage is his own type of damage, he completely ignores this assertion and affects the creature normally (no resistance).
The next time you hit this creature this turn with a weapon attack, all damage dealt by the attack becomes force damage.
The fact that this ability modifies all damage dealt to the type of force before the damage is dealt means that the weapon does not matter here, it just counts as strength damage, whatever happens.
The damage caused by the force would bypass the resistance even without the phrase B-P-S
In the basic rules of the section detailing the different types of damage, the damage by force is described as follows:
Obligate. The force is pure magic energy concentrated in a damaging form. Most effects that inflict strength damage are spells, including magic missiles and spiritual weapons.
This means that any damage caused by a force must be automatically considered magical, regardless of its source. See Force damage is considered magical if the characteristic that causes it is not magic? for further discussion on this case.
Thus, even a creature resistant to all non-magical attacks would still not resist the Planarian Warrior's attack.