The first 48 hours of a video’s life on YouTube are arguably the most important. The more views, likes, comments and engagements that a video gets in those first 48 hours affects how YouTube promotes your video in suggested videos, etc.
What they mean is that you have ALREADY optimized everything you can about the video when you upload it: titles, tags, description, thumbnail, etc. But after it goes live, that’s when you start an off-site campaign using other forms of social media to drive traffic to the video.
After I upload, I spread out my social media posts over the next 48 hours. Instagram, Facebook, Twitter, Reddit, email blast, etc. I don’t dump all that right as I upload the video. I spread it out and schedule the posts to drop sporadically over the following 48 hours of the video release. When YouTube sees that traffic, the algo (hopefully!) starts suggesting your video more due to the increase in traffic over those first 48 hours.
So, the blog doesn’t mean to upload a video and then start optimizing ON YOUTUBE during those first 2 days, but rather have all the YouTube optimization done when it’s uploaded, and then spend the next 48 hours spreading the link to your video far and wide, driving traffic to YouTube. It also means responding to EVERY comment within those first few days as well. This shows YouTube that viewers are interacting with your video and perhaps they should push it out to more people in the suggested videos.
Hope that helps!