Security through obscurity is only sweeping the problem under the rug instead of addressing it IMPO. I don't know what Parler is or was, but I don't think sequential int IDs would be the major factor that would lead to a website being scraped.
There are generally two types of information in applications, "public" and "privileged". The former has the IDs and such discoverable by an index or explore page, the latter requires authentication and has user-specific permissions.
For both cases, if hiding IDs is the only access control on the backend, it's fundamentally flawed. For both cases, if the access control is implemented well (in addition to rate limiting), integer vs UUIDs don't make a difference.
No one will argue that opaque identifiers is a sufficient security control but it's (slightly) better than nothing, at least it would've stopped a naive enumeration approach to scrape everything. Don't get distracted by that part, the main reason is many companies wouldn't want to publicly post a dashboard of how many users/entities/widgets they have for all to see but exposing sequential identifiers basically does that.
This _should_ be part of a multi-layer security plan though. You don't depend on it as a primary source of security, but why would you expose more internal information than needed? If something does go wrong with another layer that obscurity _helps mitigate the damage_.
In the case that there IS an index page that enumerates all entities you are correct. However, many systems don't provide such an index page.
In many cases it's useful for a page to be publicly accessible yet, not indexable.
This is why sites like YouTube have an "unlisted" level of permission, UUID keys are a convenient way to implement that level of access control.
UUID keys are very useful for distributed systems where a local machine wants to generate a unique key locally, and then later upload it to a centralized store. It's especially helpful in third normal form databases where often you'll need to create objects that reference each other via the primary key.
UUID v4 keys don't give away information about the number of rows in a relation. You can directly use them in api responses.
In recent events, iirc, parler was so easy to scrape precisely because they were using int keys exposed in their api get endpoints.