Sure it's a significant moment in the narrative context of the anime since its something that has not been done for 22 years, yet, I can't even force myself to even feel anything about this.
Sun and Moon in general has been such a terrible series in my eyes that it sapped away any sort of enthusiasm in me. Add to the fact that this 'league' is still very divisive, and I've maintained my opinion on how I'm not a fan that this was essentially a free for all tournament where 80% were scrubs. Battle quality ranged from mediocre to half-decent, majority were 1 vs 1 (with tons of padding), and even the semis and finals were only 2 vs 2 and 3 vs 3 which just makes the overall victory seem less significant.
Also, this league basically felt like it was tailor made for Ash, rather than a genuine league with top trainers that Ash had to overcome to prove himself as the best. And as others have said which I'll not go further into, AG, DP or XY should've really been where he won given the tone of those series, and how the league was essentially his main goal since the start, instead of being a sudden event more than halfway through the series.
So to sum up, yes, significant moment in the context of the anime, but a very underwhelming series and moment to actually do this.