By that logic, there’s nothing guaranteeing iMessage on iPhones is secure or private either because it’s closed source. If you don’t want to trust Beeper mini, you’ll be free to run their iMessage bridge on your own Matrix stack when they open source it at some point, which they’re promising to do (and you still won’t know that Apple isn’t scraping your messages on the iOS side). When I decide to trust a company, it’s because I look at what they’re transparently communicating to their end users. Every indication is that they are trying to get out of the middle of handling encrypted messages. Their first move to make this happen was allowing people to self host their own Beeper bridges (which you can still do with Beeper Cloud if you prefer and you will know that your messages are always encrypted within the Beeper infrastructure). They aren’t going to release the source for their client ever because that’s the only way they make any money.
I assume you’re not using iMessage anyway then because Apple’s Messages stack isn’t open source. If you’re not using iMessage anyway, it shouldn’t matter to you what Beeper Mini is doing. This app isn’t for the ultra paranoid. Neither is Google’s RCS in Google Messages. This is where Signal and Matrix would be better choices. If you are using iMessage on an Apple device, you’re choosing to trust Apple despite their app being closed source and you’re not choosing to trust Beeper, which is fine and I don’t judge you at all for that stance. But at that point, your qualms aren’t simply about Beeper Mini being closed source, the implication is that you don’t trust Beeper as a company and/or its developers which, again, is a valid stance even if it’s one I don’t share.
But I am personally pretty sure I can trust Beeper and Apple enough with my relatively meaningless conversations.