The sentients we fight in the game seem to be some expeditionary force led by hunhow that became sterile during the jump, compared to the ones who settled in tau and look a bit different like the shota from the trailer.
"The Sentients returned from Tau, changed... and seeking vengeance." - old war codex entry
It's uncertain how long a period of time in the orokin era is due to their lifespans, but it can be concluded that the update takes place after the first phase of the old war was over and the orokin realized ballistic weapons and swords utilized by warframes could turn the tide, resulting in the treaty. Thus it can be concluded that some disagreement on tau between the operator and the shota resulted in him being convinced by the separatists to fight the orokin, and he's possibly the one who ends up becoming hunhow, while the separatist woman in the comms ends up becoming the worm queen and also convinces the grineer to rebel, eventually leading to the second phase and the subsequent collapse of the orokin empire. The victory parade and subsequent genocide of the orokin is still a mystery but maybe it will end up being retconned.
>inb4 it's an alternate timeline
Lorelets need not apply.