It seems like a bug that even a logged in user cannot download the BigQuery connector.
I notice that the "Visit Partner Site" button is on all Qlik Market listings and usually redirects to a download page on that partner's site. The BigQuery connector just happens to be provided by QlikTech and so it redirects back to the Qlik site, however, the download page appears to have changed or is misconfigured.
It would be nice if someone from Qlik could confirm this. I don't believe the connector is meant to be available to only partners. That wouldn't make sense