There is no requirement to use the Platinum Card to purchase airline tickets in order to use the lounge access or PriorityPass benefits. Completely unrelated.
That's right. What you may be confusing this with is the requirement that you be flying on a same-day ticket on the airline operating the club for some, but not all, airlines that accept the Platinum Card for admission. This requirement does not ever apply when the Priority Pass card is used for admission.