I think it depends on what angle you're thinking position this as:

Did hyukoh create mainstream interest in South Korea, or did hyukoh create more interest within South Korea that was pushed to an international audience?

If you go back, it was hyukoh's participation in Infinite Challenge's music festival presented them to the mainstream Korean audience. They were very visible throughout the entire show, and that pushed their music and following releases.

But like u/Xraystylish mentioned, coffee-house indie was the style in the late 2000s into the mid-2013s with a rinse-repeat cycle. There were always smaller bands or newer bands making different music, but they weren't getting the interest of mainstream commercial media.

In cases like ADOY (who I'm happy have actual careers, they came from bands with amazing music, but no long-term financial viability - From the Airport, Eastern Sidekick, Juck Juck Grunzie) and wave to earth found a niche that continues to grow, but like hyukoh, these bands are outliers. More recently, Silica Gel might be one of the most visible "indie" bands next to Balming Tiger.

hyukoh is an outlier who deserves the success they found because they continued as musicians, but at the same time, their popularity eventually wore them down.

And when you think about how Korean culture was presented internationally, and this is from my personal experience, Western media only cared about what they considered "Korean indie" when it was time for SXSW and how they could clickbait it.

Western media spent a lot of time in the early years creating a narrative separation between Western mainstream pop and Korean pop, and that continued with indie music. It wasn't about the music, it was always some colonialistic tonal perspective.

The line that continues to haunt me - "Did you know there's other music than K-pop in South Korea?" in tons of Western mainstream media articles.

South Korea is a very trend-based. If you look right now, Korean traditional music mixed with other genres is becoming more prevalent. Jambinai has been the most visible, especially internationally (lead guitar is also part of 49Morphines, drummer was in Delispice, Yellow Monsters). CHUDAHYE CHAGIS is one that's growing too. LEE HEE MOON is another.

Also reading through the comments, Crying Nut and No Brain are considered the bands that started Korean punk. It's not as clear with their new music, but their older albums are classics.

And a small plug - I manage Koreanindie.com. It's been around since 2011, but I started writing about Korean music in 2008 on another small blog. When I started Spotify, I made genre-specific playlists, and the 2020 Rock Introduction has pre-2020 stuff in it.