My guess is that Greg K-H is mostly counting patches from @debian.org addresses to get his debian numbers. Since I get similar, though not identical, numbers from doing that. But there's another way to do it: Since it's easy to get a list of all current Debian developers. I generated such a list, and then grepped for commits by each name to the kernel in the past 5 years.
In total, there were 131 DDs who had committed a change to the kernel, and the total number of changes committed by all DDs was 3127.
Here are the top DD kernel committers, limited to the ones who have more commits than Greg's numbers say Ubuntu has. ;-) Just for kicks, I've included the organization they have in their email address too.
502 Roland Dreier (cisco.com)
339 Matthew Wilcox (wil.cx)
310 Roland McGrath (redhat.com)
236 Nathan Scott (aconex.com)
209 Henrique de Moraes Holschuh (eng.br)
178 Kyle McMartin (mcmartin.ca)
113 Brice Goglin (myri.com)
112 Mark Brown (sirena.org.uk)
Note 1: I suspect that the stuff Jon developed and Dave is using is much better than my stupid shell script. This is more of a back of the envelope calculation to see how a different approach can yield significantly different results. These numbers should not be trusted, used in presentations at confereneces, or used to tell people to quit their jobs. (If you do, don't put my name on the slides.)
Note 2: No attempts were made to deal with the issue that different DD and kernel committers could easily have a name collision.
Note 3: Most of these large committers are not on Debian's kernel team, which seems like Debian is missing an obvious opportunity to have a better maintained kernel.