A few possibilities:They strongly implied the time frame in the announcement, so why are they being coy now? Did they just see the fan speculation and decide to turn into the curve, in the hopes to generating discussion and buzz?
There may be a time jump between end of DSC-5 and SFA. If DSC characters will appear in S1 they really need to get this nailed down, and perhaps the current stories are still in a bit of flux. So it could still be "post" DSC but they haven't announced anything bc they haven't figured out exactly when.
They know exactly when it's going to be but they want to wait for a special occasion - perhaps SDCC or NYCC to announce it, along with a teaser or trailer.
It takes place in a completely different time period, perhaps a hundred years after Discovery, and only long lived characters would be able to cross over (unlikely IMO)
It takes place in a completely different time period and no Discovery characters will be participating, but they don't want to spill the beans until they could put together a more cohesive plot. (slightly less unlikely).
I'd personally put my money on "B" but
