Kodi isn't available in the WebOS app library, you probably need to sideload some precompiled app, or even compile it yourself. Plex doesn't need a PC to be installed on the TV, you simply use the remote to download and open the WebOS app.

Kodi is available as a WebOS app via Homebrew Channel (https://www.webosbrew.org/) and can be installed through developer mode without even having to root your TV.

If you've got some hangup about plugging a video source in your TV -- the way we've always done prior to TV vendors deciding they wanted to not just make the screen but control what's on it -- and would prefer to use a hobbled TV OS running on an underpowered SoC instead of just using a cheap and far superior Rasperry Pi, then, yes, you can indeed still just install Kodi directly onto the TV.

So it's actually worse than I've described, you have to root your TV first...

No, you don't. Re-read my previous comment.

The link you shared literally has the following content:

Title "Get Started"

First card "Root your TV"

Description "Jailbreak your TV and install Homebrew Channel. Gain more control over your TV"

So, to install Homebrew Channel, as your comment instructs, I'd first need to root my TV. No way are any non devs capable of that.

Re-read the link you've shared.

The comment that I suggested you re-read was the one where I explained that you can install Kodi from Homebrew Channel via developer mode without rooting your TV.

And the link I provided very clearly and explicitly documents both methods: rooting and dev mode.