if(!$(el).is("select")) {
    // the input field is not a select
}

$(el).not("select") gives you array. Array is always gives true in boolean expressions. But after you apply not, this array of elements won't contain selects. See working example.

The .not() method returns a new jQuery object with everything from the original object that doesn't match the selector.
You can't just use it in an if statement like that.

Instead, you use the .is method, which checks whether the element matches a selector and returns a boolean.

if (!$(el).is('select'))