A 100-person office that has one introvert coming into it every day because they are looking for some quiet time is not considered 'vacant' by this statistic.
I don't even think it would track it if 0 people were coming in, as long as the lease is still active. Commercial office leases tend to be on the longer end compared to residential (eg. 10-year is not uncommon).
The proper way to track it would be traffic in the commute hours and spending on lunches and so on during the work day, I suppose? Isn't that what cities are measuring and looking at with this return to office stuff?