Posted about it myself here:
https://testyourmight.com/threads/psa-jax-steel-patriot-skin-and-jax-klassic-arms-in-kronika-vault.71518/
Did it myself, definitely legit.
Pretty sure it'll be there for everyone whenever they visit the Krypt. It's in a fixed location and it doesn't cost any koins, unlike every other Kronika vault. I hadn't even installed the new Klassic skin pack before I got it.
EDIT: TIMED EVENT - 24 hours... That's gonna piss some people off.