Exactly, that missing information is what makes the data bad. Negative examples are needed to make any meaningful comparison between single vs team. You're missing an entire distribution showing success vs failure. You gave a perfect example of selection bias.

You would make a terrible statistician, or maybe you're highly skilled in the art of How to Lie with Statistics.

Imperfect data is a fact of life. Provided you understand what it represents ("Of startups that succeed, a large proportion have single founders") there is nothing wrong with using this.

It's much better than what you have offered: no data and insults to anyone who disagrees with you. You may want to consider this when thinking about why you have been unable to find a co-founder.