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It's not 100% certain or inevitable that all invincibly ignorant non-infants (those with the use of reason) have committed an actual mortal sin.

I happened to be clicking around the Summa last night on sections on venial sin and this was partly addressed coincidentally. St. Thomas does seem to think it impossible for venial sin to exist with original sin within one soul to the exclusion of mortal sin.

https://www.newadvent.org/summa/2089.htm#article6

“I answer that, It is impossible for venial sin to be in anyone with original sin alone, and without mortal sin. The reason for this is because before a man comes to the age of discretion, the lack of years hinders the use of reason and excuses him from mortal sin, wherefore, much more does it excuse him from venial sin, if he does anything which is such generically. But when he begins to have the use of reason, he is not entirely excused from the guilt of venial or mortal sin. Now the first thing that occurs to a man to think about then, is to deliberate about himself. And if he then direct himself to the due end, he will, by means of grace, receive the remission of original sin: whereas if he does not then direct himself to the due end, and as far as he is capable of discretion at that particular age, he will sin mortally, through not doing that which is in his power to do. Accordingly thenceforward there cannot be venial sin in him without mortal, until afterwards all sin shall have been remitted to him through grace.”

Again I just stumbled on this last night and have never put any thought into the general matter, there might be some context I’m missing leading to me misinterpreting or misapplying his writings here.

Sorry I should have been more clear. Many people I've spoken to hold that these invincible ignorant persons are somehow members of the Church by 'the soul of the church', and that they are somehow saved without baptism, which makes no sense to me.

Stemming from the same modernist humanist egalitarian assumption that all schoolchildren must be given the same trophies and gold stars, since virtue and merit are unfairly elitist and retrograde notions.

I wonder also whether, in countries that are majority Protestant, the false dichotomy of heaven-or-hell unfortunately infects the beliefs of some under-catechized Catholics, post V2 and post Hans Urs von Balthasar especially.