Yes, that's it.. after 100 hours the game will come to a stop, flash the names of Sam and Dan Houser on screen and then eject itself from your disc tray.
That number comes from the latest Houser interview where's in the midst of statement and needs to pull a number out his ass quickly. Read the interview, it's clearly not meant as a hard number. At best, it's a ballpark figure of the amount of content they aim to offer. But we have no way to know what he means. Is that the story-line played end to end, or getting a 100% on the stats screen, does it involve side missions? Does it involve looking for even the rarest, hardest collectible? Does it include failing missions and replaying them and time just spent rampaging?
You can't put a number of hours to a non-linear game. Especially when it offers a ton of replayability like this one. I'm going to obviously play each heist at least twice, since there are two approaches to each of them. Missions will be interesting to replay as a different characters anyway. Knowing Rockstar they have likely added some different dialogue when you replay something. And for me just being in that world is going to be fun, so I spend at least half my playtime doing nothing in particular. Walking or driving around and looking at things, punching random people, etc..
Both IV and RDR have gotten me well over 100 hours of gameplay