I doubt if many people mean that in quite the way you imply. I can think of two ways in which a similar comment, or even those exact words, could be meant in proper context:
- Technically Linux is just the kernel. Thus, no Linux distribution is Linux, although they all by definition include Linux. Ubuntu, Debian, Fedora, Gentoo, etc., are all collections of Linux and other stuff. This is the point of view of certain people, and it's the official position of the Debian developers. Such people generally prefer "GNU/Linux" when referring to a whole distribution, since so much of the core functionality of most Linux distributions comes from the FSF's GNU utilities.
- Because of Ubuntu's popularity in the last few years, some people have taken to using "Linux" and "Ubuntu" as if they were synonymous, but in fact Ubuntu is far from being the only Linux. I just named a few others, and in fact there are dozens (maybe hundreds) of Linux distributions around, especially if you count small or specialized tools. Linux is used on everything from cellphones to supercomputers, and even on Ubuntu's target systems, there are plenty of alternatives that are just as much Linux as Ubuntu is. Thus, the comment "Ubuntu isn't Linux" might not be meant to diminish Ubuntu's status as a Linux distribution, but rather to emphasize that Ubuntu is just one of many "flavors" of Linux distribution. This might be important if somebody is drawing incorrect generalizations about Linux based only on Ubuntu -- say, assuming that they all use APT for package management or start up X automatically when they boot.