But this league had ZERO standards. You mention the Battle Royal, but almost all of those 151 trainers were unskilled kids with like 1 Pokemon ever, who had never even battled a Kahuna, let alone beat one, let alone beating all four of them. And the Kahunas are at best average or even below average gym leaders given how weak Ash's Alola team was. When you have the likes of Jessie, James, and all the SM companions aside from Kiawe in the Alola League, the bar was basically non-existent. The only trainers with any skill whatsoever in the Alola League is Ash, Gladion, Guzma, and to a lesser extent Kiawe and Hau. The rest are all fodder. Having 150 other trainers out to get you means nothing if at least 135 of them would get dominated by an average gym leader, let alone even the weakest of legitimate league competitors.
How many of those 151 do you think could beat ANY of the average gym leaders from a past region, let alone the top-tier gym leaders like Volkner or Wulfric? Fantina would beat everyone in the Alola League aside from Ash, Guzma, Gladion, and maybe Kiawe and Hau, and she wasn't even the strongest Sinnoh gym leader. The average gym leader would curbstomp at least 135 of those 151 competitors with little difficulty, and would comfortably beat anyone outside of the Alola League's semi-finalists (and maybe Hau). Keep in mind, I've only ever mentioned the average gym leaders + Fantina/Volkner. Someone like Volkner or Wulfric (even without Mega Abomasnow) would beat everyone aside from SM Ash/Gladion, and if we count Mega Evolution, Wulfric probably could beat SM Ash or Gladion.
Even Kanto Ash with his 5 pity badges (I'd say 6 pity badges given he was handed the Blaine battle without needing to battle Blaine in another 3v3) would beat anyone in the Alola League aside from Guzma, Gladion, and maybe Kiawe. Same goes for Unova Ash. BF/DP/XY Ash would curbstomp everyone in the Alola League without question. Either Pikachu, Sceptile, Base Greninja (let alone Ash-Greninja), or Infernape would be enough to solo the Alola League. As for Volkner's badges, the likes of Barry would still pretty easily beat anyone in the Alola League 1v1 aside from SM Ash, Guzma, or Gladion. As much as people harp on about Kanto's pity badges, Kanto Ash still had a stronger team than anyone outside of the Alola League's semi-finals. Even the Furfrou guy from XY with his seven Kalos badges would certainly beat everyone in the Alola League except the semi-finalists and Hau. Even Astrid's nerfed Mega Absol as it was shown in the Kalos League is far stronger than any Alola League Pokemon not belonging to one of the semi-finalists. After all, basically everyone who lost before the semi-finals except Hau is weaker than a slightly-above average gym leader (i.e. Fantina/Clemont) and would get steamrolled by a competent gym leader (e.g. Juan, whose ace Milotic is at best slightly better than as Ash's Glalie).
TL;DR: You overestimate the challenge of the Battle Royal. Almost all of the 151 trainers were weak trainers, Ash's non-Kiawe companions included. Nobody in the Alola League would stand a chance against an decent gym leader except for the semi-finalists and Hau. Everyone else in the Alola League would lose to said gym leader, and would need a Z-Move just to not get completely stomped.