That's not what I observe, it opens in the "default app" for "navigation". I've just tried this on my iPhone, running iOS 18.5.

If I click on an address received via iMessage, it will open the "default app for navigation". If I long press it, the context menu will say "get directions" which opens the "default app", open in "google maps" if it's set as the default app. There's no option to open it in Apple Maps. If the "default app for navigation" is Apple Maps, everything I said above changes to Apple Maps.

If I click "share", Google Maps doesn't show up in the list, but neither does Apple Maps.