Instead they should be betting on the web, completely. Ubuntu, being a traditional Linux distro, has a big library of desktop apps that can be cross-compiled in Emscripten, but instead they split their much smaller resources between several different options.
Or are you suggesting that new phone OSes ought to add proprietary non-standard browser extensions in place of a native apps API? Given the ugly history of proprietary browser features it is difficult to imagine anyone taking such a strategy seriously.
So given that, if they were going to put some eggs in another basket, why would they choose Ubuntu over Android, which has an existing application base and at least some user awareness?
It seems like we're at the same stage right now in the mobile OS world as we were in the personal computer market back in the 80s, with multiple competing platforms such as the Amiga, Atari ST, BBC Micro, Commodore 64, PC, Mac, etc. Each had it's advantages, and each had significant success, but there's actually only room in the market for two or three of them long term.
I agree, Android would be a better second bet. But they don't necessarily need to limit themselves to 2 - look at how many options Samsung is keeping open for example.
Nokia screwed up, but diverting resources to an even more miniscule market is even crazier.