I think you captured definitions really well, but I'm not sure you captured the spirit of a programming idiom.

The spirit is essentially pattern-like, that the specific syntax has a higher meaning in that language such that as soon as you see its shape you know what's up. Moreover, if you don't see that shape it doesn't register as "native" for that language.

In your explanation, the spirit is "entry point is main." It's true the mechanics are unusual to Python, but that's not necessary and not all that counts. If I did the name/main comparison some other way it wouldn't be idiomatic because that very specific construct is the idiom.

Some examples in other languages that aren't that hard to read mechanically:

    param = param || default; // idiomatic for ES5 default

    let bool = !!(whatever) // idiomatic for boolean coercion in JS

    reversed_list = forward_list[::-1] # idiomatic for sequence reversal in Python

    x = x >> 1 // idiomatic for x/2 in C

In some of those cases there are practical realities that also drive them (the bitshift divide in particular comes from embedded and other speed concerns) but they aren't constructs that you see elsewhere very much outside that language even if they're valid. That's what makes them idiomatic to that language.