If you don't have 51% of the whole network (ie. more hashrate than the others), you don't have a 100% chance of executing an attack. The honest chain still has a chance of overtaking you and you have to acknowledge their blocks. The probability of success decreases with the number of blocks.
You're correct that the probability of success is less than 100% without a true 51% as you described, and my description was not accurate either. But in summary:
http://hackingdistributed.com/2013/11/04/bitcoin-is-broken/"But the outcome is that the selfish mining pool, on the whole, nullifies the work performed by the honest pool through their revelations. [...] But this threshold is non-existent in the current implementation -- selfish mining is immediately profitable. Our proposed fix raises the threshold to 25% if universally adopted. And, while there may be other fixes, no fix can raise it above 33%. So, at least 2/3rds of the Bitcoin miners have to be honest."
A lot of disruption can be caused with a 33% attack. Attacks with repeatable attack surfaces (such as depositing, withdrawing, then depositing again to an exchange) will eventually succeed. But they definitely won't be able to double spend at will or prevent the network from operating normally.
