It shows, Blender has come a long way, but FLOW doesn't look technically incredible. On the otherhand, I just rewatched Shrek recently, and complex graphics isn't everything.
Example: https://images.squarespace-cdn.com/content/v1/58586fa5ebbd1a...
There are probably some flaws here as well, but you need to study the picture in detail. And Flow used the fast renderer of Blender, not the quality one.
Still, it does have a unique style that is much more interesting than many other animated movies. So what is technically impressive, just throwing more compute at it to make it photorealistic?
I think art style will have a larger impact. In a way it is technically impressive as it didn't need a lot of compute power.
We did come a long way
But - you have to be paying attention to see it, because Pixar knew the limitation of their systems. For example, rendering of the time made everything look plasticy, so they rendered ... plastic toys as the main characters! The most noticeable graphical issues are with the rendering of people, but those are put in the background.
This movie was rendered with the latter.
Like any 3D package, you can also install other renderers.
So any perceived deficit in picture quality here is more to do with budget than some limitation of Blender.