Is it true that all invincibly ignorant non-Catholics with no mortal sins on their souls go to Purgatory after they die?
Anyone who dies invincibly ignorant of the Gospel and without mortal sin would indeed receive the purification of Purgatory for their venial sins.
Catechism of the Catholic Church:
847 …Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience - those too may achieve eternal salvation.
1260 …Every man who is ignorant of the Gospel of Christ and of his Church, but seeks the truth and does the will of God in accordance with his understanding of it, can be saved. It may be supposed that such persons would have desired Baptism explicitly if they had known its necessity.