To me that’s like saying that someone who missed Mass on Sunday, without knowing that doing so is sinful, needs to confess that they missed Mass. No they don’t - because they didn’t know it was a sin.
No such is not the same case. Someone who honestly did not have a clue that they had to go to Mass --and heck did not even know to find out – yes can be without sin via their ignorance --but then no actual sin was committed. So nothing to confess.
In the case of a person confessing mortal sins --we are referring to a different matter.
Mortal sins that where committed – were indeed
committed before the confession…
There are sins to be confessed. And being mortal --they are to be confessed “in number”.
They need to be submitted to the keys (such is the obligation). They are to be confessed in number and kind (and circumstance that changes the kind).
If a person say did not have a clue that mortal sins need to be confessed in number --well I would say they would be (all things being equal and presuming contrition and they were intending to confess all then needed to etc) be indirectly absolved with the “1” that they did actually confess of each (murder without a number is presumed to be 1 murder).
But they like other forgotten mortal sins or mortal sins otherwise omitted in a legit way (like one is dying and is given absolution and then later recovers…or one is given general absolution and later survives the plane crash) --still need to be
submitted to the keys due to nature of the sacrament (which includes a juridical nature). We need to confess --to accuse ourselves of all our mortal sins in confession and be absolved directly of them.
(now could it be that a person seeks to make an honest confession and forgets a mortal sin etc and never remembers it? sure…no worries …God knows we can totally forget things and never remember…such can still be an formally integral confession even though it is not materially integral)
More from Jimmy Akin:
jimmyakin.com/2006/09/a_reader_writes_1.html
(again I note that some which difficulties with scruples can have various difficulties --say wanting to confess all sorts of things unneeded or confess again and again out of scruple --they can be in a different boat and they need to be directed by a regular confessor)