The TfL finance committee papers are published, and it has various graphs, on page 4 is the Elizabeth line, and it seems the usage budget goes maximum in period 10 2021-22 (TfL use 13 periods of 4 weeks April-March, so is Christmas- Jan2022), for one period only
Not sure if this means a launch service has been budgeted for that date, otherwise cant see why it is so much higher than the other 4 week periods
My guess was when the budget was set (March ?) TfL Board assumed opening just before Christmas, even though now sounds like nearer May 2022
Christmas is generally a busy period on LU - it is traditionally when the record busiest days occur.