TikTok also has web URLs that they could open in the app, as do many other apps.

That’s a different situation though.

iOS supports universal links, so a website and iOS application can indicate that when you open a link to the website but have the application installed, the application opens and takes you to the content. This is what TikTok can use to open links to TikTok in the application.

But this isn’t a general purpose website => app association. Only the website owner can allow an application to do this. You wouldn’t want, say, Google to set up their application to open DuckDuckGo URLs, for instance.

So when it comes to third-party Reddit clients, they can’t automatically open reddit.com URLs because they don’t own reddit.com. The clipboard trick is a workaround for that.

It’s worth noting that if iOS allowed for setting default apps like other operating system this wouldn’t be as much of an issue because then I could always open a reddit link in the app of my choice and no longer need the clipboard workflow.