In other words, it seems like they are saying it meets VESA 600 standards, but it hasn't gotten the official certification (yet?) for whatever reason. 

I don't see where they use the VESA certified logo or the word "certified" in the AW3821dw sales page? 

For example, see the use of the word "certified" and also the VESA certification logo on the Dell S2721DGF info page: 

https://www.dell.com/en-us/shop/dell-27-gaming-monitor-s2721dgf/apd/210-axeh/monitors-monitor-accessories