How can I say “there was already a lot of salt in the meat”?
我知道你们没有放盐，但是可能因为肉是腌过的，所以对我来说还是太咸了。这不是你们的错，我仍然非常感谢你们对我的照顾（帮助）。


This translates into:


  I know you did not add salt, but, possibly because the meat was pickled, it was still too salty for me. This is not your fault. I still thank you very much for the care you have shown me.


Hence, this is a polite and considerate way of addressing the issue.