So I could be wrong about their point because they used SQL schema as their comparison but the commenter does question typography specifically so I think this is more "why is topography so focused on?" not "why is design so focused on?". At least that's the more interesting question to me.
For example this page. First thing you see is a hero image showing their font in use. It's a pretty important element that takes up the whole screen. It's blurry and pixelated. Not the worst I've seen but definitely gives me more car dealership not car manufacturer vibes. It's a 1.3MB png that looks just as good converted to an 80KB jpeg and imo even better as a 20KB avif. If it was instead a 1.3MB jpeg or avif starting from a high quality source that thing would look crisp and super car manufacturery.
There is also a section in the page where the text goes blurry because they've included a table as an image. Or I should rather say it's both blurry and an image because they could have just included a higher res image of the table. They have plenty of other images where it looks fine. Again something that gives me car dealership vibes. Actually I would say blurry text image table is specifically a dodgy second hand car dealership level of vibe.
Now you could say that the typography page isn't user or customer facing so its not that important but I see plenty of shitty blurry images on otherwise polished looking pages all over the internet, and also if they covered images in their design system like they do typography then it wouldn't matter. They'd just be in the habit of only using high quality images and have some process in place to deliver them in appropriate formats.
So while backend vs frontend or functionality vs polish are imo boring and played out webdev topics, I am super curious as to why they care enough about inspiring positive feelings through web design to make an entire custom font, have a page of guidelines on how to use said font, but dont have the same level of care for images. And why I see this contrast enough to consider it reasonably common in the industry.