As per the article, they’re not legal tender so no one is required to take them as payment.
I think the phrase "legal tender" has a lot to answer for here. It's a really well known phrase, and you'll hear shop owners say they can't accept these notes because they're not "legal tender", but the actual definition of it is only applicable when settling debt in specific circumstances.
It's quite entertaining to handover a Danish-Irish-UK pound sterling note in the UK, like somehow the viking era never ended.
Though last time I was in London, I dumped a few coins from my currency stash for a museum entrance donation and I didn't otherwise touch my cash at all. And don't think I touched my Euros at all when in Dublin for the same trip.
I’ve had Scottish bank notes declined in the past though.
The local Fish & Chip shops only accept cash, so it does help me to avoid eating unhealthy food, although if there was a way to find it when they’re using fresh oil they’d probably be far more tasty.