Why would Apple configure MacBook Pro to use inferior graphics card when a better one is installed?
MacBook Pro has come in models with both an integrated and a discrete graphics processing unit for many years now.

The integrated graphics uses less energy than the discrete GPU, so in order to maximize battery life, it is used whenever possible.

The integrated graphics should be plenty powerful to play ordinary YouTube videos or play a DVD, which are the tasks you describe in your linked question. Unless you have an external monitor connected to the MacBook Pro, the integrated graphics should handle this fine.

However, you can definitely configure your MacBook Pro to use the discrete graphics. This way you can test whether or not the problem goes away when using the discrete graphics.

You can the setting by opening System Preferences > Energy Saver, and removing the checkmark from the "Automatic graphics switching" checkbox. When this checkmark is removed, the computer will use the discrete graphics at all times.

Read more about it in Apple's documentation: 

https://support.apple.com/en-us/HT202043