I've seen the slab - who can resist a side trip to a genuine flying saucer crash site (according to the plaque)?
I don't enough about concrete and rebar to know how accurate the attempts have been to determine its age or to link it to the time frame Aztec was alleged to have happened. Is it any more accurate than dating Kodachrome?
As one who hikes a fair amount, often off trail, I have seen a fair number of slabs in both probable and improbable locations. I don't think it's possible to determine its use at this point - when was it laid, what was the land used for, what were the roads like, who is there who could possibly remember? It seems more probable to think it was used by a rancher or oil than it was as part of a crash retrieval. Some think it might have been a natural gas well plug.
Still, it gave me something fun to track down at the site - in addition to the plaque, the alien head rocks, and a geocache. But as far as artifacts go, I think the rusty cans found at the alleged Kingman crash site are more interesting. Ultimately, though, any such "evidence" only shows that somebody was there doing something and does not get us very far.