The Placement Editor included with AppGameKit certainly looks and feels like it was made with AGK. It has the same icon and folder structure.
So, it is very likely that the answer to your first question is: YES, it was made with AGK.
On a side note, as far as I can tell, the Placement Editor binary has never been updated since AppGameKit 1076 (or possibly earlier). So.. it has propably been made with a very early version of AGK.
I think there have been a few previous questions about the Placement Editor source, which have gone largely unanswered, just like your question.
However.... in my opinion,
the Placement Editor included with AppGameKit is utterly horrible! (In fact, it is such crap that it should not be included with a product like AppGameKit at all! It is just completely substandard!) Not only is it badly designed and lacking several important features, but it's also severely buggy. There are cases when you can build and save a "world" with it, but you cannot even load that savefile to modify it further (so, under some specific conditions, it fails to load it's own savefiles).
I wish I had kept my savefile when I discovered this bug, but I didn't keep it, and I have very quickly (and a long time ago) came to the conclusion that the Placement Editor just doesn't meet my requirements - and built my own editor tools.
Also, there is a community project that has aimed at creating an enhanced version of the Placement Editor, from scratch. You'll find it here:
Placement Editor Professional by David Hynd (aka
Clonkex).
(This one has the full AppGameKit TIER1 source code included.)
Cheers,
AgentSam