If you want air quotes around compatible, I can add them. However the entire point remains.

It is GPL compatible because the GPL licenses can explicitly be added to it. Adding the license requires all of the restrictions that licence brings.

Is your entire point that GPL implements copyleft?

I took the pint as saying that Apache/MIT/BSD/MPL code that is distributed as part of a GPL project is actually under GPL. To obtain the original Apache/MIT/BSD/MPL code you need to get it from upstream.

So that if project A originally licensed as MIT is statically linked inside a project B licensed under the GPL then in the event that all copies of project A source code are destroied except those distributed via project B (a more likely scenario before mirror servers) then project A is now only available under GPL.

I believe that GP relies on the fact that GPL source code can only be statically linked with other GPL code and so "compatibile with GPL" means "can be relicensed under GPL" rather than "can be included as is in GPL works"

My original comment and point has not changed. The GPL is only compatible with the GPL. The only way for a license to be compatible with the GPL is to have the ability to license under the GPL. We have proven that is the case.

There is a lack of awareness and understanding of how this can transitively affect things accidentally.

For the record I am all for GPL code. I think the GPL is a fantastic license that has enabled great good in the world. But there are reasons why a company may not want to release code that is compatible with it.