I wonder why that app needed a relational database for vector tiles in the first place. That seems like read-only data that could be heavily cached at the edge, too. I'm not surprised that Scylla outperforms CockroachDB there, though.
It probably has something to do with the difficulty of maintaining a worldwide spatial index otherwise - it's really, really hard to keep the index consistent and up-to-date with a bespoke product, and every time I've worked on GIS the solution for more than a few square km of data has been "stick it in a database so we can index it."
But tiles don't need to be spatially indexed — they're offsets on a 2D plane in a Mercator projection. I've only worked with bitmap tiles, but it's my understanding that vector tiles use the same system. So all you need is fast (x, y) lookup.
I've used PostGIS extensively. You don't need PostGIS to serve tiles unless you're deriving the tile data from a relational database and need to convert the tile coordinates to geographic coordinates.