retraction? (Eruvin 43a)

This is a summary of the gemara in Eruvin 42b-43a courtesy of kollel iyun hadaf.

Question: All agree that the Halachah follows him [Rabban Gamliel] regarding a boat - what is the reason?
Answer #1 (Rabah): It is because he was Shoves within Mechitzos from before Shabbos;
Answer #2 (R. Zeira): It is because the boat constantly moves him out of his four Amos (he is like one who was forcibly taken by Nochrim out of his four Amos, he receives the four Amos in which they put him - this applies every time he steps, therefore the entire boat is permitted).
Question: What is the difference between these answers?
Answer #1: They argue about if the boat's walls were diminished [from 10 Tefachim - Rabah's reason no longer applies, R. Zeira's does];
Answer #2: They argue about one who jumped from one boat to another (Rabah forbids, for he was not Shoves within these Mechitzos).
Question: Why didn't R. Zeira answer like Rabah?
Answer: The walls of a boat are merely to keep the water out (they are not considered walls to make a boat like four Amos).
Question: Why didn't Rabah answer like R. Zeira?
Answer: All the Tana'im permit a moving boat [for R. Zeira's reason], they argue only about a stationary boat (R. Zeira's reason does not apply then. Tosfos - we retract from the two answers given above regarding the difference between Rabah and R. Zeira.)

When I went to the point by point summary I was happy to see that they quoted Tosafos because that's what I wanted to write about. Tosafos says that it must be that we're retracting because Rabba agrees with R' Zeira by mehaleches. R' Akiva agrees that if the ship is moving then you're allowed to move throughout the boat but he only argues by a standing ship then his reason must be mechitzos. Tosafos seems to assume that if R' Akiva agrees in a moving ship because of the walls then R' Gamliel must hold of that sevara also and therefore the gemara must be retracting the first two answers.

I don't think that it's muchrach to say as Tosafos does. It's true that Rabba agrees with R' Zeira that R' Akiva's reason is because the boat is moving but that doesn't necessarily mean that he's changing his explanation of R' Gamliel. He could be saying that is exactly the machlokes between the two. Rabban Gamliel says that the reason is because the mechitzos were there and R' Akiva says the reason is because the boat is moving. That means that R' Akiva could end up being more meikil. He would say that if the walls fall down or if someone jumped ship that you would still keep get 2000 amos but if the boat is stationary then you only get four amos. Rabban Gamliel would say just the opposite that if the boat is stationary you still get 2000 but if the walls fall down or you jump ship then you don't get them.

I haven't had a chance to look at the other rishonim but I suspect that someone must say it because pashut pshat in the gemara is that we're not being chozeir from what we said originally.