Let's do the math.

CX has 3 flights a day to LHR. This time of year they're running close to 90% full. That's close to 1,000 passengers a day stranded since the airspace closure, all waiting to fly.

Once CX starts flying to LHR again, it's going to take weeks to accommodate the thousands by filling every empty seat that's not taken by people who are already ticketed to fly on operating flights.

The May 7th estimate is probably accurate.