U can always "Ubuntu" Ubuntu
This is what is great about open source. If Ubuntu takes all this great software code, and makes it easier to use on their OS, Redhat, etc, can turn around and make their distro that much easier to use too.
Redhat I think is at fault themselves for retreating into a niche. They saw Enterprise support contracts as their main revenue source, and appeared to the market to have abandoned any plans at being a significant player in the home-user desktop market.
So 50%.... well first off assume this is common speech, and not meant to be a scientific fact.
It's not entirely unreasonable (especially from their work-day perspective) that people at Cannonical could feel that half of what comes out of their OS is their work. It also depends on how you weigh the "total package" that is the OS.
Say you bought a Cell phone - One ergonomic engineer may think that the phone is 90% his device, and that the software only counts for 10%. The software programmer probably feels the same in reverse about the silly hardware that runs his precious code, a matter of perspective.
If Ubuntu is easier to use/install for average users, that is a major value add, and a real tangible benefit.
So is 90% of the OS the underlying Kernel?
How much would each application account for?
As much as the coders may think otherwise: marketing, presentation, packaging, ergonomics, easy of use, etc. are all EXTREMELY important factors to the average PC user. MUCH more so than the software code, or even security. The average user cares about who wrote the code in their machine as much as they care who painted the numbers on their keyboard (hint: they don't ever think about either).
Ubuntu has done major contributions to Linux with the development of their OS. If Debian, RedHat really have that much work-hours going into desktops, they should be able to freely adopt all of Ubuntu's work also.
That is the nature of a community project people!