The issue is that they didn't start treatment earlier because no one except Erin knew that the mushrooms were foraged.
The is no risk of Amatoxin Poisonings with mushrooms from supermarket or Chinese grocers because Commercial mushrooms are grown in huge warehouses. Deathcaps cannot be propagated because they rely on the roots of Oak trees.
Copying this over from the previous thread -
It's been reported that Erin was an experienced forager and that the family foraged together:
A family friend reported that Erin Patterson was known to be good at foraging wild mushrooms, and that the Patterson family would “pick mushrooms together each year when they were in season”
If it wasn't intentional, I imagine she would have described to her guests how she'd put so much effort into foraging for local mushrooms for the dish. There'd be no reason to hide it, her family would be excited about eating the locally hand picked delights.
Likewise, when the guests presented to hospital the morning after the lunch, they'd have reported to hospital staff that there were locally foraged mushrooms in the dish.
As Deathcaps are known to grow locally in the area (and cannot be cultivated), then amatoxin poisoning from foraged mushrooms would be seen as an immediate possibility - it wouldn't require waiting to hear Erin's lies about where the mushrooms came from and they'd be able to begin preventative treatment in case amatoxins had been ingested, instead of waiting for signs of metabolic acidosis and starting the treatment WAY too late.
Early treatment is critical, that's why - As soon as amatoxin was suggested by the toxicologist as a possibility - they wanted to immediately start the kids on preventative liver treatment - even though they didn't have symptoms.