hello: The Rashba is referring to circumstances where a Siman is a valid solution, namely in the home, and even there its uniqueness must conclusively indicate its status.
I disagree strongly with your understanding of the Rashba. As you have quoted him above
In my opinion, the Rashba holds that the bread can be kosher if it is seen as “questionable”. Meaning, that when someone sees the bread, they will come to question either a)the purpose of the siman or
b) the status of it whether it’s dairy or meat.
That is what he is referring to in the last line, that a person will see a siman and will come to ask and be told.
If I live in a house where home made corn bread is regarded as a questionable product, I am fine according to the Rashba, because I will come to ask someone and will get the right answer.