Bug? An ObjectExpression's key isn't an identifier

I think there’s a mistake in the AST of acornjs when parsing an ObjectsExpression.
For example, for this source code:
({foo: 2 })
Returns this:

{
        "type": "ObjectExpression",
        "start": 1,
        "end": 10,
        "properties": [
          {
            "type": "Property",
            "start": 2,
            "end": 8,
            "method": false,
            "shorthand": false,
            "computed": false,
            "key": {
              "type": "Identifier",
              "start": 2,
              "end": 5,
              "name": "foo"
            },
            "value": {
              "type": "Literal",
              "start": 7,
              "end": 8,
              "value": 2,
              "raw": "2"
            },
            "kind": "init"
          }

It says that foo is an Identifier but it can’t be possible as you can’t do ({foo: 2 })[foo] but you can do ({foo: 2 })["foo"]. So {alert:alert} == { "alert" :alert}
Instead, if it was a real Identifier, it should be ({ [foo] : 2 })

This is how ESTree specifies the AST should look like.