The Rishonim on the Sugya ask your question. Tosfos on 6a (DH v'Lechi) explain that the Lechi itself must be made wide enough that the distance from the end of it to the other side of the street is less than ten Amos. The Gaon Ya'akov asks, however, that this does not solve the problem for Korah. Perhaps we could say that since the street is more than sixteen Amos wide, and the Korah must be over a gap of ten or less, the Korah will have to be placed on a wall protruding into the street, reducing the width of the street at the point of the Korah. This protrusion will not have a Din of a Lechi since it is more than four Amos wide, and the Gemara says (5a) that a Lechi of more than four Amos is not considered a Lechi. The Ya'avetz explains that if he places the wall in order for it to be a Lechi, it can be considered a Lechi, as explained in Rashi and Tosfos on 5b.

Tosfos on 6b, DH v'Chi, offer two different explanations. Firstly, Tosfos suggest that whilst the Gemara thought that we are talking about a Reshus ha'Rabim, it did not realise that a Lechi does not work for more than ten Amos (or thirteen and a third according to Rebbi Yehudah). In the end, the Gemara says that the Beraysa refers to a Mavoy, not a Reshus ha'Rabim. Secondly, Tosfos say that the street itself is not sixteen Amos wide, but is considered a Reshus ha'Rabim because there is a Reshus ha'Rabim on either side of it that leads through the Mavoy. This being the case there is no problem putting up a Lechi or Korah.

Dov Freedman