Garden socket which IP certification?

The Schuko plug of outdoor devices is designed to fill the entire socket and thus seal it somewhat.

No, or only minimally.

Or am I misinformed?

Yes.

Have you ever plugged such a plug into an outdoor socket outside of forum theory?

This here is the plug of the Hue IP67 power supply in an IP44 socket. It doesn’t seal anything beyond light drizzle: