In response to the question indicated below take a look at the Stiepler, Kehilas Yaakov, Eruvin 3. He goes in depth as to the pshat in the Ritva.Chidushei HaRan also discusses this same issue.
My understanding of the Stiepler, which was also explained at my Daf-Yomi shiur, is that when there is an "existing mechitzah that has a fault of "Gedi'in Bok'in Bo" that is a psul in the mechitza - since one cannot even begin to apply the law of "Gud Achis". The mechitza never has a point in which it is complete. (i.e.Actual Reality)
However in the case of "PI TIKRAH YORED V'SOSEM" the law of "Pi Tikrah Yored v'Sosem" IS first applied and the "virtual" mechitza has now already been put in place. At this point one cannot apply the fault of "Gedi'in Bok'in Bo" since the "vitual" wall already has been fully created. (i.e Vitual Reality)
The Stiepler does end off with a Tzarich Iyun. (The location of the Stiepler is Kehilas Yakov, Eruvin #1.)
Simcha Blatter
Thank you for pointing out that piece in the Kehilas Yakov. As you mention, though, the Stiepler himself ends of with a Tzarich Iyun. His explanation does not explain why "Pi Tikrah Yored v'Sosem" works differently (so that the Mechitzah is formed before the Beki'as Gedi'im comes). The explanation which we gave helps a little to understand why this is.
(There is some support for our explanation that the Tikrah creates a Heker which serves to mark off the area underneath as a separate domain from the Gemara on 14a. The Gemara says that when a mat is spread over a Korah, the Korah is invalid. It seems from here that "Pi Tikrah...," too, would not work in this case since there is no longer a Heker for the area under the ceiling, when it is covered.)
M. KORNFELD