Having switched outlets wouldn't make US plugs any safer, at least not in any meaningful way.
The individual switches on UK outlets don't really add significantly to safety, they're mostly just a convenience feature, because for an electrical plug/outlet to even be considered safe in the first place, it has to be always safe, whether it's powered or not. You can't rely on people switching off unused outlets instead of doing actual safety design.
The main factors that make US plugs less safe than UK ones is the potential for exposed metal contacts with a closed connection to the outlet, the lack of internal fuse and the lack of polarisation, and, particularly in combination with the first point, the comparatively weak grip strength and protruding design that make it easy for a plug to become (partially) unplugged by accident.
They are referencing the lack of isolation on the prongs for US plugs. If a US plug isn't fully inserted, it's possible for both of the two prongs to form electrical connections with the outlet, whilst not yet being fully inserted.
This means a small part of the prongs which are now at 110V potential to each other is exposed, and could potentially be touched by a child, or cause a short circuit if an object gets into the gap.
So yeah, the electrical code in the US for household plugs is just straight unsafe.
You can see the way to do it properly in this post: Notice how the two L+N prongs only have exposed metal at the very tip, this, if they're inserted deep enough to create contact, it's not possible for any exposed metal to still protrude from the outlet.