The part in Before the Storm where the Goblins attack the Explorer’s League camp come after the events in game, where the SI7 is sent in to find out what the Goblins are up to and take them out.
I don’t recall any SI:7 in Silithus in Before the Storm. In fact, the SI:7 were getting their information from the Cenarion Circle members that were down there.
The champion is sent to Sillithus after Anduin’s speech. This occurs before the attack on the Explorer’s League camp. But no, they don’t mention any SI:7 in Silithus, to say nothing of what they might be doing there.
The champion is sent to Silithus a while after Anduin’s speech was over. It was presented as flashback, and the gathering of people had already cleared out when the Alliance player talks to Shaw.
They script the encounter so the Horde player opens the door for Sylvanas, a door she couldn’t get open herself. The cinematic fight between Sylvanas and Genn plays out. When its over Sylvanas tells the Horde Player to their face that she will continue to hunt Eyir afterward as well even if she has to attack the Halls of Valor itself.
The completion text for “A New Life for Undeath” the Horde Quest this involves.
This includes Horde Warriors who by this time have thrown their lot in with Odyn and Eyir with the Class Order Hall.
Horde players were not going to stop her and the warriors are actively throwing their class order hall under the bus for her.
If the story, plot, universe, and its characters bend to the will of a character it can be classified as a “Mary Sue”… which is exactly what Sylvanas is. A terribly annoying one at that.