The original Tenchu with more than 10 missions and new characters and timed missions? Nah, man. I don't know anything about this game but the achievements alone betray any notion that it's the PS1 Tenchu.
[ Gamertag: OMGaBEAR ]
Reached 10000: July '08.
20000: 4/29/09, 26th birthday gift to myself.
30000: Oh, you know what? I forgot. Done counting.