The reference you've cited is for sharing within your primary SF Org, not within your Community. You'll need to set-up additional sharing for your community which might include enabling External Org Wide Defaults. Primary resources you'll want to look at are Getting Started with Communities, the Security Workbook, the Security Implementation Guide and possibly A Guide to Sharing Architecture as well.
With a partner community, everything is going to begin with how Accounts are shared since Opportunities and Contacts are both children of Accounts. You may need to revisit your sharing model from the ground up in order to achieve your desired results, both internally and externally.
The first thing I'd recommend you look at is whether the related activity is visible internal to Org when viewed from Opportunity. If it's not, then perhaps there's an issue with the WhatID on the Activity which could be the root cause of your problem. I don't know if you use Opportunity Teams or not, but that may be something you'll need to implement and then set-up sharing around those to allow Community Users who are on those Teams to see view everything related to their Accounts and Opportunities from the Community side of things. You may also need to look at Partner Profiles to see if they have all the necessary permissions enabled that are available to them. This is a many pronged approach to solving your problem.
In essence, you're going to need to study the materials I've referred you to in order to come up with a solution that works for your Org and Community. Its very difficult, if not impossible, for someone like me to give you an answer that will solve your problem without being able to look at your Org and see how everything is configured. The best I can do is point you in the direction of where you might want to look. Ultimately, I believe you're going to need to create sharing groups. You may find that what will work best for your org is to disable External Org Wide Defaults and only use Sharing Groups, which is how this was all done with Communities and Portals until fairly recently.