If it were trying to match people based on their personalities and not their direct line of thinking - you would be correct.
Until the results speak for themselves (ie. accuracy of matches) you have no way of knowing if this is a complete failure or successful venture. Although you are free to speculate that it will be meaningless due to the methodology, you have no evidence to back that up yet.
Many dating sites work on a similar principle, although they all use weighted questions. Weighted questions for personality increase possible matches by narrowing down the preciseness necessary. The goal here is to decrease possible matches by requiring exact matches.
Dating sites want to match you with someone quickly who is "close enough" to be compatible based on a personality test and their user data of what answers pair well with other peoples' answers.
8x8 doesn't care how long it takes to find a match, they want an "exact match" to be compatible based on how you answer arbitrary questions.
8 questions was chosen because to have answers be "100% exact" are already rare. If they chose 50 questions - the chances of an exact match become increasingly rare as to possibly never exist in a persons' life time.
E:
The birthday paradox is 1/365, and I'm well aware of the statistics of someone out of a group of 20 random people sharing a birthday with another person of that same group.