I found out that all R1C1 references are treated as relative references. So "R1C1" which whould be "$A$1" is instead treated as "R[1]C[1]" which means "cell one row below one column to the right"
use avadim\FastExcelWriter\Excel;
$excel = Excel::create(['Sheet 1']);
$sheet = $excel->sheet();
$sheet->writeRow([1,2,'=R1C1+R1C2']);
I found out that all R1C1 references are treated as relative references. So "R1C1" which whould be "$A$1" is instead treated as "R[1]C[1]" which means "cell one row below one column to the right"
Like if I wrote
=R[1]C[1]+R[1]C[2]