Hi Don,
The previous and next images are switched
It’s not, previous means the post published earlier than the current post, not the previous post visually on blog page.
The method from the topic you linked was before we introduced dynamic element to GP Premium
I would recommend to use a block element – post navigation template instead of the complicated PHP and CSS method.
https://docs.generatepress.com/article/block-element-post-navigation/
The video in the attached video shows how to create custom post navigation.
Please be noted, GenerateBlocks plugin is required to use the dynamic options in GP.