Wrong assumptions can lead to wrong conclusions.
that probably these closed numbers belonged to the people who died
That assumes that each closed number belonged to one person (or maybe two numbers per person).
In China there exists (existed?) what are called "Click farms".
As per Yahoo Finance there can be hundreds of phones in one such Click Farm.
possibly attributed to cover up of deaths?
No, because as the Bloomberg article you link to points out, the likely explanation is that people are canceling extra phone plans. They may no longer be able to use extra phones due to changing economic conditions or travel restrictions.