IANAL, but I think it comes down to your interpretation of section 4:

> You may convey verbatim copies of the Program's source code as you

> receive it, in any medium, provided that you conspicuously and

> appropriately publish on each copy an appropriate copyright notice;

> keep intact all notices stating that this License and any

> non-permissive terms added in accord with section 7 apply to the code;

> keep intact all notices of the absence of any warranty; and give all

> recipients a copy of this License along with the Program.

So by removing the copyright notice from "each copy" of the "convey"ed source code, I think you could argue they violated this.

But as others have argued, even if you don't believe that this section forbids it, the polite thing to do would be to either a) get permission to remove the copyright notices, or at the very least b) create a file (e.g. CONTRIBUTORS.md, or a section of the readme) and place the copyright information in there. Saying "Ooh, we don't like X's name at the top of the file, it's too ugly" without any attempt to maintain some acknowledgement might not be a violation, but certainly isn't within the spirit of the AGPL, the main point of which is to "(1) assert copyright on the software".