Seen as the readme says this in the very next sentence, and they reference Flash, I think they’re actually talking about the full fat Oracle Java runtime, not just Javascript.

Side note: I found a support page on speedtest.net that still says you need Flash installed, but only Flash, no Java required here: https://sandboxsupport.speedtest.net/hc/en-us/articles/202610754-What-are-the-requirements-to-use-Speedtest-net