That IBM website promoting it is the most ridiculously overengineered yet totally unusable website I've used in quite some time on desktop. It's really quite something. Also disappointing that the Github README doesn't include alphanumeric sample text.
Eventually I just gave up and found a usable website to actually view and sample the font: https://fonts.google.com/specimen/IBM+Plex+Mono
Luckily, the font is much better than the website!