Those people in the control room just see various things on their screens (see the attached image). It is quite evident that not all the monitors show the same data.
NASA JPL Mission Control Room, shortly after a woman said "Touchdown confirmed."
Watch this video, especially after minute 53.
However, it is a bit baffling that immediately, seconds after the moment a voice said "Touchdown confirmed", most of the people demobilized, left their sits and never returned to their computers.
It would have been more logical to rise their hands, applaud, stand up (manifest their joy) for a few moments but after that they should have returned in front of the monitors had they really supervised data sent by InSight.
What if something bad had happened minutes after the landing like a minor fault in the beginning that would have slowly evolved in a catastrophic failure unless a prompt command from earth had fix the problem?!
It looks to me that most of those people there were a kind of extras. They had no real role in controlling and supervising InSight.