damage resistance Billet, piercing, blow of non-magic attacks
When a Statblock says so, it means that the creature is only resistant to the damage types of cudgel damage, puncture damage, and damage done by non-magic attacks. Because it is a damage of its own, it ignores this statement completely and affects the creature as normal (no resistance).
The next time you hit this creature with a weapon attack, the entire damage dealt by the attack becomes Force damage
The fact that this ability alters any damage to the Force Force before dealing damage means that the weapon does not matter here, but only counts as Power Damage, no matter what.
The power damage would bypass the resistance without the BPS phrase
The basic rules in the section dealing with each type of damage describe the damage as follows:
Power. Power is pure magical energy concentrated in a harmful form. Most effects causing power damage are spells, including magic missiles and spiritual weapons.
This means that any power damage, regardless of the source, should automatically be considered magical.
Even a creature resistant to all non-magical attacks would not resist the Planetary Warrior's attack.