This is hilarious. This is one of those "in the right place at the right time" things. Vandalism like that usually only is up for a few minutes, apparently the same few minutes when DuckDuckGo was crawling that page. Amazing.
I don't have experience with this sort of thing yet, but I'm going through something similar. I don't think it would be unreasonable for you to ask to see their earnings. Both you and the start-up are making a large investment and deserve to know as much about the other as possible. The likelihood that they will be successful, considering you will be employee number 4/5 will largely depend on you.
I disagree with the your dichotomy. It isn't whether the end-user likes the design, it is whether the design is useful. So useful that they don't even realize "design" is taking place.