Soklin on the holdings

Hello Rabbi,
I heard in your lesson on doubt and statistics that what they said "Soklin on assumptions" means that we have not entered into doubt at all, and therefore we go with what we know and do not begin to be satisfied from nowhere (perhaps someone who is held as our son is not our son, etc.), but if we truly had doubt, we would not have decided to die based on assumptions and things would settle down in the heart.
But the G.M. in Gittin 17 says:
Itamar: Why did they fix a time in Gittith? Rabbi Yochanan said: Because of his sister's daughter.
And in the same place:
Because of his sister's daughter. He should not cover her when he does not have time, unless we say that the maintenance of a man's wife was established and he married her after divorcing her, since she was divorced before us, and even more so, the maintenance of a kosher woman who was not married when she was a man's wife was established.
So we see here from the text that even in the case of doubt that we do not know when it was sold, we would still judge it positively based on presumption?
(If it weren't for a side reason, but it seems like the entire discussion is a discussion of assumptions even when we have doubts)
Thank you very much Rabbi for all the wonderful content you upload.