The verb “to be” is infinitive. It indicates the present. If Trump’s tweet’s grammar had been correct, it would have been ‘Much has already been learned”. This is not present tense but present perfect tense, and the verb “to be” becomes “has been”. The meaning is different.
“Already” indicates an indeterminate time in the past, and here it compliments the meaning of the present perfect tense by indicating that something happened in the indeterminate past and that the condition continues into the present.
I think this tweet error is highlighting the missing [en] in be[en].