https://html.spec.whatwg.org/multipage/embedded-content.html#the-iframe-element says:

When an iframe element is inserted into a document that has a browsing context, the user agent must create a new browsing context, set the element's nested browsing context to the newly-created browsing context, and then process the iframe attributes for the "first time".

We are currently just checking that the iframe is in a document, not that the document has a browsing context.