There was some speculation floating around after the MrHappy interview that, since the dev team expected a negative blowback after the gender-lock reveal, that they may have wanted to make both sets of players happy but may have gotten a big no from SE, or a "you get one last race". So they intended to weaponize player reaction and force SE to allocate them a bigger budget. The wording of "last races" allows them to bring the missing genders later down the line as a "we said last races, and we meant it, so thanks to player demand here's male Viera and female Hrothgar!"

That is, of course, purely speculation to be taken with a grain of salt.

As for the WHEN, that, of course, relies on manpower and time. I do think 6.0 isn't too far a stretch, but that's assuming they start in 5.1 like they (apparently) did for fViera and mHroth, although that still involved a lot of time on the planning and design stages (eg. Viera were not decided to be the counterpart to Hroth until later, probably a bit before or after Ridoranna Lighthouse). In any way, I'm cautiously optimistic but not really trying to get my hopes of. After all, there's nothing official yet.