(Sure, a flight near the Canadian or Mexican border could in theory divert to a Canadian or Mexican airport, but it's unlikely many people on those flights have their passports with them.)
In which case freezing domestic (and the huge amount of load it contributes to ATC) but keeping international normal makes sense.
Of course... the side effect is the "fun" that creates for incoming international flights with domestic connections...
There must be some procedures in place for this - I pretty regularly take domestic (Canadian) flights that pass over the US where I don't carry my passport.
Flight EY8 from Boston to Abu Dhabi https://fr24.com/ETD6PX/429055e2
The intent around the ground stop is to reduce in-air U.S. traffic.
As for asking already airborne flights to turn around, those coming in over the ocean might simply not have a non-US alternate airport available with their fuel remaining, so that's often not an option.
Oceanic flights are planned so that at every point along the route, the aircraft can lose an engine or cabin pressure and still reach a pre-approved diversion airport with enough fuel, with the flight plan marking the points where turning back versus continuing becomes the faster option. If a diversion is needed mid-ocean, the crew offsets from their assigned track and altitude to stay clear of other traffic, then heads to the nearest suitable airport, such as Gander, Keflavik or Shannon over the Atlantic, or Anchorage, Cold Bay or Midway over the Pacific.
Alternate airports are picked based with regards to all kinds of contingencies (many of them meteorological), but "the US closes literally every airport" is not one of them.
So my guess this is what it means "case by case". Some which are close to destination would be manually routed, others will land in different places.
Depending on the nature of the disruption it may be they simply have drastically reduced capacity to handle traffic so landings may be much slower and they'll manage the traffic based on various priorities.