His objection (I paraphrase).
"There is no Scripture in Scripture, that says Scripture alone is sufficient for faith and practice".
In logic, this is called a self-reference. It is a statement that speaks about the statement itself.
Here is another example:
"This statement is a lie".
Our language is capable of self-reference, but such referencing is not capable of being evaluated. It goes into cycles. So in the above, if that statement is false, then it is true and is not a lie, which it said it is etc etc, you can go crazy at this.
So going back to the requirement of the RC apologist, he seems to be saying that in order for sola scriptura to be true, you should find a statement of it in Scripture. There is one in Scripture such as 2Tim 3:16-17 that matches this but this is dismissed by RC apologists because it is not explicit enough.
There is a fallacy going on in the apologist's requirement. There is no necessity that for a document to be true, it has to state something about its own truthfulness and if it does not, then it is not adequate to inform.
Another point is this, even if there is one such explicit statement, the apologist can come back and say - well self-reference is not valid anyway because you are simply stating your statement is true, does not make it true.
Sophistry is like that, your system is able to prove or refute too much. At first blush they think this is virtue, but as I often say - this is not virtue, instead it is vile. A form of reasoning that is able to prove anything you like is not a good thing because that means you have falsehood also inside it.
Ex falso quodlibet - from a contradiction (falsehood) you can deduce anything. This is the fallacy that sophistry includes and is hidding inside that form of reasoning and for this reason is able to prove anything.
As fallen people, we are capable of sophistry. Even in culture such as the pop culture of young peole, you can observe this happening today.
When people are swimming in their own sophistry, they are not aware of it or fanatical adherence makes them deny it. It is like telling the fish - hey do you know you are swimming in water? Huh, what water?