You are given access to the WS in the BC regardless whether or not you've unlocked it. Similarly, once you have the weapon you automatically have access to it even if you have never unlocked it.

SE intended for mythic WSs to be tied to mythic weapons like Relic/Empyrean WSs are, but they recognized that:
1) People weren't doing Nyzul Isle as much as they wanted
2) It would be very unpopular to release another set of exclusive WSs when no new WSs had been released in several years

So they decided against it and gave mythic weapons to all the jobs. This is very obvious if you look at Pyrrhic Kleos (three lines per swing, one for each of PK's blades).