That is only one part of it though, and probably one of the smaller parts. The main reason he should be trusted is because he owns bustabit, and previously owned moneypot. Moneypot had lots of bitcoin invested in the bankroll, and I don't believe a single satoshi was stolen since after the sale, people could access and withdraw their funds at mp.bustabit.com. Bustabit has also had a lot of bitcoin wagered, and large winners are paid out very quickly, with some huge 10+ bitcoin withdraws. There wouldn't be much of a point for Ryan to create a site, just to scam people since if he did, his site (which probably makes him nice profit) would get a lot smaller volume, and there would be less profit.
Yeah, I think at a single point of time I had held >1000 BTC of other peoples money that I could've stolen if dishonest, and in total processed a little over 16k BTC of peoples withdraws. It doesn't really prove I will always act ethically, but bithra.com is just supposed to be a light weight alternative to "Send me the money first", not the be all and end all. I specifically avoided any advanced features and usage just to make it simple.
The use case I'm trying to replace is: "Hey, I'm new and selling X. Please send me the money first, I promise I won't scam you. I don't have time to use a human escrow". It's not trying to replace trustless on-chain schemes, and things that offer better guarantees.