1.0 is not an upgrade, it's the first stable release. Usually it signifies the arrival of some amount of feature completeness and stability compared to the fast paced 0.x days. Of course semver doesn't really fit neatly most software let alone a user facing GUI application, but socially that's what they're trying to communicate with the 1.0.
These days those versioning is just PR and doesn't mean much like if something is stable. Gmail used to have beta mark for how many years but was still used. Rect native is 0.79 but doesn't mean it's not production ready.