With an ergonomic chair, sitting upright will not require frequent reminders from yourself. Design encourages correct posture via the chair.
To many people, sitting upright feels quite tiring, even though that’s usually because it is of a poorly designed chair. Shoulders tighten up, your lower back feels some pressure, as slouching or leaning forward is encouraged by a seat that is poorly designed. Your posture suffers as well, and in due time, your muscles will strain.
An ergonomic chair helps you sit well so that is fixed. It can prevent slouching, so it keeps your body in alignment plus it supports your shape naturally without pressure. The outcome includes better posture with effortlessness and reduced tension. The day requires less reminding now.
So yes, an ergonomic chair improves posture indeed. It does so not through forcing a different sitting position but by making the correct one most comfortable.