But wouldn't the efreet urn and the treasure chests also detect as magic (and evil for the efreet)? And they actually are magic (and evil). So, in a room with 8 things to mess with, 7 of them will detect as magic, 5 will detect as evil -- but only 3 are actually magical and only 1 is actually evil. Of the 4 false magic/evil things, 1 is the key to move forward in the Tomb. And the party has no way of determining what is true magic/evil and what is giving a false positive.
I'm not sure what being detectable as both magic and evil but not actually being magic or evil has to do with anything. The statues are pretty much the only things in the Tomb that could possibly be "iron men of visage grim" so it's very unlikely that would be misinterpreted here. But then, right after perhaps the most conclusive line of the poem (!) it tells you to go LEFT twice. So why would any party bother looking at the statue to the RIGHT of the door? After the requisite number of Transformer comments (they are "more than meets the eye") most party's will likely waste hours examining the east ("left") wall.