Speculation: Chinese zoom users were required to switch to Chinese version of Zoom last September. There were probably mainland users using international version + VPN to participate in the event, probably some sort of local regulation that prevents users in CN from interacting with international meets, especially on no-no subjects. From 2nd link below:

>>Intensifying international tensions and the country’s upcoming 70th anniversary are cited as reasons for the block, according to Chinese media.

Some old articles:

Zoom suspends Chinese individuals users from hosting meetings due to ‘regulatory demand’ https://technode.com/2020/05/15/zoom-suspends-chinese-indivi...

China’s Zoom users switch to local version after blockage https://technode.com/2019/09/19/chinas-zoom-users-switch-to-...

China blocks US video-conferencing tool Zoom https://technode.com/2019/09/09/china-blocks-us-video-confer...