Silvermoon is a Horde capital. There is no reason why Alliance should be allowed there.
While Alliance allied High Elves that aren’t Ren’dorei (because the void is a danger to the Sunwell) are allowed pilgrimage to the isle of Quel’danas (the Sunwell), Silvermoon City is not a place that welcomes members of the Alliance and nothing has changed in that regard.
All the new customization options allow is for Alliance players to play as the Alliance allied High Elves they see in Alliance capitals and other Alliance friendly locations. Silvermoon is not an Alliance friendly location, nor should it be.
Silvermoon is a Horde city. Any Thalassian that has not declared themselves as citizens of Silvermoon, and thus members of the Horde, have no claim upon it.