Opensource software is proprietary at the same time?

I couldn't upload a picture of it here because it doesn't let me post it but the software app in debian shows proton.deb and cpupower-gui.deb and some other .deb files which are supposed to be opensource as "Potentially Unsafe provided by a third party; Proprietary code" and " Proprietary This software is not developed in the open, so only its developers know how it works. It may be insecure in ways that are hard to detect, and it may change …

app code debian files gui opensource party privacy proton software third upload