Please understand that Ubuntu specifically patches the firefox and chromium to add back their vendor string and that makes them a bit of a rare duck.
firefox as shipping in fedora no longer lists a vendor at all
firefox as shipped in opensuse no longer lists a vendor at all
And to really bake your noodle... some versions of stock linux mint firefox... lists ubuntu..because they didnt change the UA string patch ubuntu includes in their debs..so you can't even separate out mint clients from ubuntu clients in the wikimedia stats. Is mint or ubuntu more in use in the desktop now? wikimedia stats cant tell you for sure. Can't say from those stats. And I know some people in both camps who would love to be able to say one way or the other for sure.
I've looked at this a lot, fired up a lot of live images across multiple distributions checking the ua strings. The rise of the unknown linux is wikimedia is very complex. Its partly due to chrome showing up, but thats not the only factor. There are multiple ways to rationalize the unknown linux that are not supportable by the facts in hand. The rise of unknown linux and the fact that its the largest linux vendor in the stats and has been for over a year now really can only be reasonable lead to the conclusion that you can't rely on the ua string to tell linux vendor.
The size of the unknown linux category represents the uncertainty in the vendor-ness of linux clients quite broadly. At best you can sum up all the non-android linux vendors in the stats together and say that according to wikimedia non-android linux is stagnant percentage of client marketshare year to year...with some monthly variation.. there is some seasonal ebb and flow...but long term...pretty stagnant. Only android has really seen any real market percentage from a wikimedia clients pov since i started trending the stats.
Even if you take the very whimsical view that all the unknown linux is one vendor, of your choosing.... that one isn't gaining market share over the last couple of years. So meh. All the growth is iOS and Android. Not much really to talk about beyond that.