This is a mistake. The classic case of kol kavua is where you have a piece of meat in your hand and are not sure which store you bought it from; not that you do not know which store is kosher.
You are right; it was a mistake. Now, having looked over that Tosafos and looking up Tosafos in Zvachim I see that he definitely says that the Issur and the Heter should be discernible. That is what makes the Issur Kavua.
As to your original question, I think the point is simply that if you had been close up you would know who is the Jew, in whatever way — perhaps by asking him or by his clothing, or by knowing him.
A good way to understand what Kavua is, is to compare it to Asham, were we are Mechalek between two pieces of meat, on of which is Cheilev, and one piece of meat that is a Safek if it is Cheilev. We use the term Ischazek Issura. The point is that were there are two pieces, the Issur is a reality, but when there is one piece the Issur is a Tzad in your Safek.
The same can be applied to Teisha Chanuyos. If you find a piece of meat it has nine Tzidadim of Kashrus and one Tzad of Tarfus. If your Shayla is on the stores, they are ISchazek — the Issur store is a reality sitting right there.
So, Tosafos is saying that when there is one big mix, of animals, people, or rings, the Issur — being merely a Tzad — is Battul. On the other hand, when the Issur is “right there” it is Is’chazek, or in our terms, Kavua.