Idiom: the language peculiar to a people or to a district, community, or class

http://www.merriam-webster.com/dictionary/idiom
1 a: the language peculiar to a people or to a district, community, or class : dialect b: the syntactical, grammatical, or structural form peculiar to a language
2: an expression in the usage of a language that is peculiar to itself either grammatically (as no, it wasn't me) or in having a meaning that cannot be derived from the conjoined meanings of its elements (as Monday week for "the Monday a week after next Monday")
3 : a style or form of artistic expression that is characteristic of an individual, a period or movement, or a medium or instrument <the modern jazz idiom>; broadly : manner, style <a new culinary idiom>
[M-W's Col. Dic.]

1: What is a 'structural form'? Isn't 'structure' and 'syntax' the same?

2: 'no, it wasn't me', what is so much idiomatic about this one? It's a meaningful grammatical sentence in itself. Perhaps, it's 'it', used for animals and non-living entities, which is making up for peculiarity.