But how do they know what day is that? Unless you have already something ready to be announced (and you just hold it until the very last moment, which doesn’t make sense, since you could just announce it asap)

It can also be ”we are done but wanna test it more and tweak it” and then ”oh they launched now. Let’s launch then as well”

All the people who are any good at AI talk to each other. There's no secrets among those who are making 7 figures plus in this field.

"keep refining and testing it until we're really done or somebody else releases"

Maybe a little corporate espionage.

Probably more keeping an eye on the behavior of the competition and predicting what they might do and adjusting your own schedules.