Apple TV and NAS with iTunes Server?

No (Despite what any manufacturer my advertise)

what is it they are add with?

do it show up in itunes as a shared lib but cant be used with Atv?

did it used to work but no longer with new itunes and atv firmware?

or are they just making stuff up or mean one can access the NAT and the songs on it and add them to itunes playlist?

The basic problem is the use of the term 'itunes server', whilst not specifically claiming to do anything it can't, it implies it can do something other drives can't and that is simply not true. Some advertising goes further than this and even mentions the tv. Some of the advertising that other posters have linked to in this forum has been nothing other than dishonest, but to clarify the position I think this is more an issue with resellers than manufacturers.

but in theory it could be possible to make a external hd thingy which had anough software to act the role of a pc with itunes
in the eyes of the aTv
but it would cost and would have to have some fancy software
which had to be updated to match itunes / aTV once in awhile

That "thingy" would be called a computer, I'm afraid.

1) A lot of NAS devices are computers
2) Don't buy NAS for serving iTunes, buy it for backing up your files.
3) You can consolidate you library on the NAS device, but you'll introduce significant latency. If you do this, you probably won't be able to stream movies via wifi connection and syncing will take a bit longer.