Cannonical's statement seems odd to me on some levels. You could search & replace "C#" and "Mono" with "FAT32" and "LFN" and get a statement which could've been said in the past and we know is wrong now. It's not clear what makes C#/Mono different (though
maybe they are different; I don't know).
On the other hand, perhaps Cannonical don't see it as their job to get in the way of what people use unless someone takes legal action. They can't stop people using Mono; all they can do is choose whether or not to include it in any builds by default (which
in turn may encourage/discourage its use by people who hope to get their software included in the same builds, but that's about it). Whether or not important software for the Linux community depends on Mono is up to the community, not Cannonical, so their
statement makes sense on that level.
And yet, Ubuntu still has FAT32 drivers included by default; including Mono should seem less risky than that.