You are still confusing terms.
There are the Standards that define the protocols and you have implementations in equipment.
A standard is a document it only talks about the requirements to function. The 802.11ac standard requires 80mhz of bandwidth which means it can not run on the 2.4g band...at least legally. This means the standard itself is not backward compatible because you can not run 802.11ac on 2.4g.
Now some piece of hardware may implement the 802.11ac standard AND the 802.11g,b,n standards. That particular implementation of hardware would be backward compatible BUT the standard itself is not.