If your cat likes sardines (what cat wouldn't?) then okay as long as you consider it a treat to be given once in a while.
If your cat thinks sardines are, as Jackson Galaxy calls it, the "Jackpot Treat" then, maybe, you can use them for training your cat but, just like Cookie Monster says, they are a "sometimes food."
As for salt... Personally, I like to keep my salt levels down, mainly because I don't like too much salt and, with anything, moderation is key. I don't like it when people proselytize about eating salt and, when people take hearsay and treat it like fact, it peeves me off.
The "War on Salt" is just that. It's hearsay being treated like fact and it is wrong.