This has been discussed before. XT is an altcoin, and services that move from Bitcoin to XT are altcoin services. On bitcointalk.org, altcoins and altcoin services belong in the Alt Cryptocurrencies section. On /r/Bitcoin, altcoins and altcoin services aren't allowed.
An altcoin that uses the same merkle root is by it's very nature just a forked blockchain... and the shortest chain would become the altcoin and die. So, you can call it an altcoin but it's not an altcoin, altcoins have their own unique merkle root. It's just a BIP, if it gains majority consensus it's Bitcoin and if it doesn't then it's nothing.
Basically, any and all Bitcoin hardforks would be considered altcoins by your definition.
So, just calling it an altcoin is an excuse to silence any and all discussion related to a blocksize change.