SFOS 4.5.0 switches RPM compression. What does that mean for existing RPMs?

Yes that’s about right. /home/.system is a place where system-wide data is kept on the home partition, without becoming inaccessible when another user logs in.

Btw, the addition of the new compression format for rpms is a reproducible builds thing. I think the old one wasn’t always producing the same result, which meant that builds were being triggered when they shouldn’t be, and deltarpm didn’t work very well.