They were probably expecting for one of three rolls to go our way during pursuit. And didn't know that lining up a shot would take quite that long either.
There's no real line of sight between the area in front of the gates and the area behind it.
Even if no goblin touches him, how long do you think that a blind Skaven can survive within their own society, let alone remain in a relevant position? That Moulder has effectively been taken off the table.
Despite any difference in racial competence I don't see how even an expert dwarven ranger can sneak through those gates without magic. They are literally the front door of a greenskin stronghold.
Look at how many competing rolls we had to fail for the results to be as "bad" as they were.
Overall I am satisfied with the results. Losing one scouting option was a bit sad, but I fully expected that we could have been so unlucky as to achieve nothing more than a single clean kill of a prominent goblin.
[X] Having successfully escaped, you can now move on...
-[X] To the King's Gates, to scout the outer defences.
-[X] To the Temple of Grungni, to check if the holy place remains undesecrated.
-[X] To the Chiselwards, an enormous warren of tunnels that could house any number of horrors.
The armory definitely is either closed or looted so long ago as to not be imminently relevant to the battle. The furthest reaching depths seem a weird narrative choice for someone on a time limit.
My preference is the King's Gate because even if we find a target of opportunity that costs us a bit of time, we would still be right there at where we want to exit anyway. But I am okay with the other two as well.
[X] You've finished scouting, you'll return to the Vanguard...
-[X] By returning to the secondary peak, and taking the opportunity to further sabotage the Doom Divers on the way out.
This one is the most boring and uninformative option, but I kind of agree with those who say that bad artillery in an entrenched high ground position is still worse for us than no artillery whatsoever.