I re-checked how the entire onboarding process works and it looks like this is by design: the forums does need to collect your consent for both information processing as well as for the email communication. If you don't tick both boxes you are indeed taken back to the consent page again, but with an additional error explanation message displayed on top. (Perhaps it's color scheme is not the best and therefore easy to miss):

The reason for the requirement is because there are situations where forums engine will automatically send you notification emails (for example when someone replies to your topic or to a topic you watch).

Rest assured that this does not give us consent to send you marketing communication or any communication about any other Opera product/service apart from Opera forums itself.

You can select which types of notifications to receive via email, via your user settings page. To open it click on your avatar picture in the top menu and then select "Settings".

Best regards,
Artur