I've heard this before, but I don't buy it. I have been using Linux for 20 years now; back in the day, and back when Ubuntu came out I was using it on my desktop (today, not so much) and I simply not believe this claim to be true. It's often repeated, but never with concrete specific examples on what was more polished and how it was more polished.
In fact I believe the opposite to be true. Back then I was using Debian on my desktop. I naturally tried Ubuntu and found out it was less polished because it contained more bugs.