If the Ohloh's engine isn't wrong, what is the explanation of this?
The Oloh engine is way wrong. Ubuntu is a quite complex project, and it's difficult to state what is Ubuntu and what is not. But C# is not by any measure the most popular language.
Ubuntu has also much more than 50 contributors and 1500 commits/year even if you only count Ubuntu specific patches.
This site seems to follow some strange mix of Ubuntu and non-Ubuntu projects, it doesn't make sense. It's likely than F-Spot (a big program that is not in the default install base now and it's made in C#) is weighting heavy on that statistic.
The Linux Kernel (which is the core of Ubuntu) is written mostly in C and a little parts in assembly languages. And many of the applications are written in python or C or C++. C# is not a popular language for many linux users/developers because it was meant to be used for Windows. But still you are free to use it. Its just that people mostly give preference to languages other than C#.
I have seen the source code of Linux kernel and I can be 100% sure that 99% is C. You can also download the source code from the official site of linux kernel. The site is http://www.kernel.org . There you will mostly see ".h" which are referred to as "Header" files and ".c" which are "C Programming" files.
So the article you are reading is wrong. And by wrong I mean dead wrong. Please notify the site's admin for the changes also.
Ubuntu is definetely NOT written in C#! Ubuntu does not even (as far as i know) ship with any C# / Mono applications installed although they are available in software centre (banshee etc)
Ubuntu like most distros is made up of alot of C and C++, mix in some vala, python, perl and many others. C# is a bit of a sore point due to MS/.net connections.