Ho Ho ho, our two british friends never ventured to the poinsettias native range in old Mexico. Wild poinsettias have this common trait to all plants, they don't move much. And, like all plants there are these really wierd lifeforms that do, often accompanied by rude behaviour such as eating plants.
And to that plants respond with thorns or needles or, poisons.
The native poinsettia is indeed toxic enough to cause severe intenstinal distress in herbivores. It's that white milky fluid where alkaloids are stored that gives them that bad taste.
It is the non native planting soils and fertilisers that make commercial poinsettias less dangerous than their wild cousins.
But, make no mistake, household pets will chew on poinsettias, bad taste or not. it's more a territorial behaviour than a silent plea for more chicken in gravy feast.
And while ( usually) not life threatening, it can cause severe intestinal upset in cats and small dogs still.An older, or weaker animal can indeed succumb from associated stress and dehydration if not treated.
I can tell you one true fact that is no myth.
We have to many Penn And Teller 's running around as self appointed 'myth busters' the originals in Northern California themselves prone to error on occasion.