I can’t say that I understand your remark in view of the original statement and Papal answer.
Certainly,
passionate kissing is a sexual pleasure. It is arousing and therefore serves as foreplay, which is ordinate for a married couple intending to couple, and is inordinate for the unmarried. Catechism:
2350 Those who are engaged to marry are called to live chastity in continence. They should see in this time of testing a discovery of mutual respect, an apprenticeship in fidelity, and the hope of receiving one another from God. They should reserve for marriage the expressions of affection that belong to married love. They will help each other grow in chastity.
2350 Those who are engaged to marry are called to live chastity in continence. They should see in this time of testing a discovery of mutual respect, an apprenticeship in fidelity, and the hope of receiving one another from God. They should reserve for marriage the expressions of affection that belong to married love. They will help each other grow in chastity.
2351 Lust is disordered desire for or inordinate enjoyment of sexual pleasure. Sexual pleasure is morally disordered when sought for itself, isolated from its procreative and unitive purposes.
The original statement:“Passioniate kissing is also gravely sinful for the unmarried: this was actually defined by a Pope, and he condemned all other views.”
Question:Would you mind providing a source for this claim?
The answer in the linked thread

ope Alexander VII condemned the idea that it is only a venial sin for the unmarried to kiss for the sensual pleasure arriving from the kiss even if there is no danger of further consent of going even farther.
“…condemned. What does that mean? Saint Alphonsus explains. ‘Quote
this means that every time someone with sufficient reflection and full consent of the will delights in carnal or sensual pleasure associated with to whom he is not married he commits a mortal sin. This is not only true with kisses but also with respect to other touches performed for carnal pleasure. The reason is that any delight taken in carnal pleasure, that is to say any delight taken in stirring up the appetites which surround the creative power, is a movement towards the martial act.’
Close Quote. The Doctor of Moral Theology of the Universal Church.”