This is because the staff consider the bitcoin-XT currency to be an altcoin
They are correct. Bitcoin XT is not altcoin yet, but will be when hardfork of bitcoin will inevitably occur as a result of larger block size accepted in future!
Exactly.
So why censor it now? beucase I can go and download the Bitcoin-Xt client and use it with Bitcoin Core clients, so right now it obviously isn't an altcoin, its a usable Bitcoin client just like any other.
It seems that here the problem is another one, the 'money'.