However when I access it on Firefox + umatrix with all js disabled, it doesn't.
I suspect I know what it is. Possibly noscript sends some header saying that the browser doesn't have JS which causes ddg to respond with the redirect, whereas umatrix just breaks the whole thing entirely.
EDIT: I just tried going to about:config and setting javascript.enabled to false, and indeed now I get redirected. This has significantly improved my view of ddg.
Be sure you have the setting "Spoof <noscript> tags" enabled if you want noscript tags to be rendered when 1st-party scripts are blocked. It's a per-site setting, but you can enable it globally in the "Settings" pane in the dashboard.
Thank you so much for umatrix! It really has changed the way I use the internet.