Who says jinbei didn't train? Jinbei was miles ahead of zoro pre timeskip, even with 2 years training it's not guaranteed zoro surpassed jinbei.
Jinbei knew he would join the strawhats soon so i doubt he just sat on in his ass. He probably prepared for that
All of the strawhats trained during the timeskip. Jinbei is still stronger than all of them except for luffy.
Jinbei wasn't a strawhat pretimeskip, and Jinbei was miles ahead of Luffy pre timeskip too. That is obviously not the case any more.
All the strawhats were explicitly shown getting the message to go train for two years. Jinbei was not shown planning to do that, and in the time between he had to serve big mom in order to keep fishman island safe (as he was no longer a shichibukai). He couldn't have spent the time training even if he wanted to. While I'm sure he got somewhat stronger over time, like every character does in one piece between arcs,
You said Jinbei should be stronger than zoro. There is nothing suggesting that should be the case, other than tales from your ass (tm). Zoro got to fight the second in command on Fishman island (and not jinbei) for reason. Jinbei ended up paired with sanji, so I'd likely assume from there he is at or just above sanjis level.