Was it an ommission for balance to not allow sights on melta weapons?

Xeno Mesh armor in RT is AP 3 in DH it is 4. I have seen some stats for it elsewhere in RT being 4. But it is not in the RT errata being 4.

Last question.. I see people mention RAW here.. new to the board and sorta new to the new system (used to play the old RT over nearly 20 years ago).

Presumably a design choice, as it's consistent across all sight types.

I've always assumed it's a typo, although it's possible that the xeno-mesh that the NPCs you might have seen wearing it is Best quality.

It means Rules as Written. Used in reference to the specific wording of rules. Contrast with Rules as Intended.