I wouldn’t trust any third party claims for something as sensitive as this. You’re giving them a lot of access and trust and that’s not great for something that will directly talk with the iCloud where your passwords are stored.

Your best bet is to export those passwords to another password wallet that you trust that works in Linux.

You can access the iCloud through iCloud.com and I believe you can access passwords there, but of course it doesn’t offer the same convenience that it does on Apple devices.