There have been 0 matches with over 12,000 tests. Assuming their claim of 1/1,000~ is anywhere near remotely true - then you may have increased your chances of finding a 'good match' tenfold already with the assumption that matches will be good (there is of course the chance they won't be)
The Myers-Briggs places weight on questions to give one of the 16 personality types made of 8 traits. For example, one answer might give you 80% of INTJ and 20% INTP and by factoring the weights across all the questions will tell you if you were weighted more or less towards INTJ or INTP, even if you were more INTJ for that singular question.
In 8x8 the questions are not weighted in that way, which is an important difference. While there are 16 distinct personality traits in MBTI, there are only 8 traits. Questions can weigh towards those 8 traits in various ways. From my experience MBTI tests range from the 5 "strongly, disagree, neutral, agree, strongly" answer types and the questions are targeted to 2 (sometimes 4, but typically 2) traits.
Meaning if neutral is 50% : 50% (Not leaning towards introvert or extrovert) your answer to the question is effectively meaningless. However if you "agree" it might be 75% : 25% and if you "strongly agree" it could be 100% : 0%. You take the aggravated ratings of all the "I or E" type questions and determine if the user is introverted or extroverted based on which holds more weight across 50 questions.
8x8 does not weight the questions to determine personality traits. The questions must match exactly to determine a match. In comparison, Myers-Briggs can have two people be INTJ who answered questions very differently but their aggregated weights of each question added up to them both being an INTJ.
Again: this should be treated as a case study. Since questions aren't weighted, this is different than most personality tests. This isn't a personality test - but finding someone who thinks exactly as you do in regards to these 8 questions.
The largest flaw is if these are the wrong 8 questions to ask and if matches lean towards "good" or not. Which can be determined after enough matches. Making this a neat case study. :)
E:
TL;DR If 12,000 people took a MBTI I guarantee there would be many matches already. This isn't a personality test - this is a line of thinking test. Whether the questions will result in good matches or not will be left to when there are any matches to begin with.