First of all congratulations on your purchase! Is that the coin you won? Definitely a nice foundation of your (hopefully) future collection!
As regards slabbing of these coins, I noticed that Spink does this often even for not so special coins and quite likely this is to appeal to US market, but here it surely serves also another purpose, that is, to guarantee the Ellerby Hoard origin. Therefore I think it's only positive if they sent the coins for relabelling and NGC's special "Ellerby Area Hoard" label looks really nice! So I would be happy about that, even if it means delays, which of course is not ideal.
Regarding "the conversion of ‘Details’ to ‘Genuine’" I am not very sure what they mean, as NGC does not have 'Genuine' labels to my knowledge. If a coin gets Details grade, it is still geniune, the same genuine as if it got a regular numerical grade. So, not sure what that is about.
Edit: I have just realised NGC has 'Genuine' labels for El Cazador shipwreck coins and likely also for other such coins where their provenance is important. On those labels they do not state grade, just certify the provenance. So presumably that's what it's about - NGC already assigned Details grade to some of those coins but on the final Ellerby Hoard labels they will not mention Details grade, just Genuine?