defer and DOM Ready clarification

[jaybytez]

In the post: https://community.tealiumiq.com/t5/Developers/Defer-loading-of-not-bundled-tags/td-p/35391

The response makes it seem like DOM Ready is the equivalent to defer, but my question is when the Tealium IQ tags appear on the web site, DOM Ready does not add the defer attribute to the bundled script, instead it has the async attribute.  If it does not add the defer attribute, how is it essentially implementing the defer logic?