Sounds weird. But Most likely delta is telling the truth. They also cancelled all flights to Shanghai for the next week.
Yes they were.
I checked. The rule change was a stricter requirement to clean the airplane.
That results in longer cleaning time.
That results in the plane has to be on the ground for more than 3 hours
That results in the crew have to be quarantined for 2 weeks because they stay more than 3 hours.
That was not acceptable to Delta.