If you only do the grove gate fight the ending cinematic only mention you discovered the Grove and the Tielfings (which you did), then it skip to the dark fortress part.
If you watch the video from the start, you'll see how he bypassed the grove gate fight.
I think that line and the fortress line are the start and finish trigger with all the permutations in between, but maybe you can get a boat -> fortress narration if you skip the grove through sequence breaking?
The guy skipped the grove fight by never triggering it via jumping across paths. The ending script recognized not doing the Grove fight so he never meet the druids and Tieflings and tus the two unavoidable parts are mentioned in the cinematic: prologue and the dark fortress.
The lines in that cinematic aren't necessarily in the order you did them too from my experience.
