Dear HG.

I assume you were surprised that the Gemara considered peppers, etc. as possibly being the real Esrog.

The Ritva explains that the Gemara never doubted what is the real Esrog. There was no doubt to what was done since the time of Moshe but the Gemara was interested in identifying and proving from the Written Law what the Esrog is.

Also, see the introduction of the Rambam to Misnayos who writes "these proofs were not brought because there was confusion until they knew there proofs, since we saw from Yehoshuua until now that every year an Esrog was taken with the Lulav and there is no argument, but they investigated the "hint" of the Written Law".

All the best,

Reuven Weiner