Mmm, well yes, happening now. From the "Google" app made a search, a long touch on the Directions link shows a maps.google.com URL, Google Maps is installed, even reinstalled GM thinking the registration was corrupted somehow. It opens in Apple Maps. Uninstalled Apple Maps and it prompt me to reinstall.

Perhaps since this is the Google app the URL they show for long touch is not actually what is getting executed when you tap, maybe they are sending a routing command directly, but either way there's a number of UI "nudges" to push you to Apple Maps over Google Maps that you cannot adjust or turn off without jailbreaking the phone.

I just clicked on a direction from Google on Safari and it took me to Google Maps.

I did the same from Chrome. It took me to Google Maps

I went to a third party web site within Chrome, click on "get directions", it took me to Google Maps.