The above tweet is obviously suggesting that millions of people in China are dying of Coronavirus. And apparently China Mobile has some way of detecting when people are dead, and unsubscribing them.
The link goes to this article
https://ezone.ulifestyle.com.hk/article/2596539/23 年首次大幅下降 中國移動流失 725 萬用戶惹猜疑
The article raises the coronavirus suggestion, and the "official story"
They link to two sources, the first being Sina Technology:
The second source is by a facebook user, Leesimon.hk, which does not add much:
Sina Tech has two other articles on the topic:
So it seems the reasons are largely just economics. Changes in pricing structure, combined with new freedoms to port your number to other networks. Quite possibly compounded by local economic downturns due to movement restrictions around Coronavirus. But nothing to suggest they all died.