For HTTP/1 and HTTP/1.1 connections, for a single hostname, a browser is only able to download one file at a time. For some sites, (with lots of large images for example) it's possible to get marginal page speed improvements by splitting the images accross multiple hostnames so the browser would be able to download some portion of the assets in parallel.
However HTTP/2 supports multiplexing so the browser can download multiple files in parallel. With HTTP/2, having images spread accross multiple hostnames can actually be slower, since there is additional overhead from having to connect to multiple servers.
We serve all site assets with HTTP/2 - so you should be seeing images being loaded in parallel already.
The tool you're using there appears to be out of date since it doesn't take HTTP 2 into account.
Hope that helps answer your question, let me know if you have any more questions about it!