This is true, and I think it speaks to a deeper problem: nobody at Twitter really seems to understand exactly why the core product was successful, so they're afraid to evolve it out of fear that they'll somehow break the magic spell. When ancillary products like Moments are developed, though, the designers and developers feel freer to be bold, because there's no spell there to break. So when those products get bolted on to the side of the main product, they feel exactly like that -- bolted on.
Facebook has been much more fearless about changing their core product as their userbase has grown and changed -- remember when the News Feed was a controversial new thing? -- and as a result they can give their products a unified feeling that Twitter can't.