The problem isn’t new tariffs, but how the USA wants to collect them. It’s mentioned in the article:
“IMAG's Ms Muth said the overarching concern is that many postal carriers are not set up to ‘collect and remit’ the duties specified by Donald Trump's executive order.”
Normally tariffs are collected by the receiving country when a package arrives. Trump wants foreign countries’ postal carriers to collect US tariffs and somehow remit the money to the American authorities… But there are no systems set up for this. The Americans haven’t even provided a way to send those remittances.
Obviously this is not something that postal carriers around the world can just spin up in two weeks, just because the Americans suddenly decided they want foreign post offices to collect their import taxes. So the only option is not to ship to America at all.