It's not a new idea, here's an article from 20 years ago:

https://www.sciencedirect.com/science/article/abs/pii/S01685...

I can't find a good source but it seems that the radiation shielding properties of polyethylene were discovered some time after WWII. But I think this was an inevitable discovery, not some sort of accident. HDPE has a lot of hydrogen atoms which are perfect for blocking high energy particles without creating secondary radiation. A block of solid hydrogen would be ideal but that's not practical. Water works well but it's heavy and needs a container which adds more weight.

Dense metals and concrete do a fine job but are heavy relative to amount you need to stop the same amount of radiation as HDPE. Plus you get the secondary radiation effects from metals that can harm humans or sensitive electronics. It's like being behind armor that's hit with a round. The armor may stop the round from getting through but very hot fragments of the backside of the armor (spalling) can fly off and injure someone. Now you need additional protection from the spalling too.

Here's a photo from 2017 of a NASA facility using white HDPE sheets to attenuate high energy particles:

https://www.flickr.com/photos/brookhavenlab/33642244296/in/a...