You can vote online prior to whatever deadline you've been wrongly assuming.
I voted weeks ago. You said one can only vote on 12/19 @ 10.
The online proxyvote says the deadline is 12/18/2023 @ 12:59PM eastern.
However, there are alternative methods, such as attending the meeting and voting that way, which is 12/19.
The letter specifically stated to make sure to vote BEFORE 12/19 to ensure all votes are counted and tallied.
That does not exclude voting on 12/19 by attending the meeting, which is also an option.