Well I saw your replay, and really, it doesn't prove anything.
Probably there is some mistakes in this game.
EQ was useless turn 11. Archen's switch was a bad idea, imo he needed to sacrify one mon, or take a risk like switch at least vulpix on a wow.
Here, he just lose his sure fletch switch in, then he has just checks, who is basically pretty difficult.
His team has huge troubles against ponyta and larvesta once archen is gone, that's why play with him and sacrify a BJ wasn't imo a good thing.
Keep with Snubbul turn 16 was also bad imo, morning sun was p obvious, and he was in troubles, switch on sprout was an possible idea in order to change the game.
Then, Larvesta has a free Flare Blitz who is really powerful. Once again, he decides to switch on foo, and sprout was an idea.
I think probably that the player who have the sun doesn't take enough risks in this game. Suns, like all offense playstyles, requires risks.
I don't know if his team has rocks, but if he doesn't have, well it can explain maybe a part of the game. Sturdy, Fletchling, Fire Types, Vullaby... For me rocks are almost a must have in a sun, they are most useful than in all any others teams. However I admits that EndureJuice magne make harder the jobs of suns once sleep clause is activated. But just imagine with rocks the same match. Larvesta can't come, fletch can be in troubles, EndureJuice can't switch on spout's weather ball and has even troubles to deal with bellsprout. Not including stealth rocks in the teambuilding is probably a mistake if he did.
And well, I have won a lot of matches with sun against very good players, it doesn't shows that the playstyle is broken, as you can't say "sun isn't s" with one replay.
"but it's never seemed overpowering." Probably, but is the misdrea era, i never had problem with, and he was S, to take a random exemple. Maybe I was too optimist with Offense and Stall, but really, you can't say that sun loses against a lot of playstyles, bc that's not true. Bellsprout is a thing who can 2HKO like all the metagame or almost, and OHKO a lot of things in the current metagame, like a lot of stallmons (Slowpoke/Foongus/Ferro/Spritz) and let sleeping a lot of others, without counting offense mons who are almost all OHKO and outspeed.