If it's not open source, how do you know it's a privacy-respecting tool?
Because of their GT&C, I would at least hope that companies follow the law, and that the law is rather clear. This tool in particular (according to my google search) seems to be rather popular on HN, which increases my confidence a bit.
So... faith? I'm not accusing them of anything. Maybe they aren't harvesting your info.
I'm just pointing out that maybe you're asking the wrong questions.