Idk, this was a comment left by u/chrismessina on the r/Fediverse version of this post:

I find their explanation for why they didn’t build on open social web protocols baffling:

Why isn’t UpScrolled a decentralized platform?

Because it doesn’t work for what we’re trying to build. UpScrolled isn’t decentralized (yet) because today’s open protocols (e.g., ActivityPub, AT Protocol) don’t reliably deliver what we need for a mainstream, video-forward app: fast global discovery, stable search/ranking, and smooth media. In practice, these stacks still lean on centralized indexing to work well, so we’re shipping the experience that works now-not a theory.

We keep things open and simple with lightweight, common-sense protections-spam/bot filtering, straightforward reporting, and consistent deletes when people remove their own content. We’re building interoperable by design (clean exports, stable APIs) and will add optional bridges to open protocols as they mature, so you can reach more people without sacrificing speed or simplicity.

(They’ve since scrubbed this from their FAQ).