Well, looks like with some brute force, I've figured out what the problem is.
This issue happens when the code I have was setting the cell type to a string using setDataType(PHPExcel_Cell_DataType::TYPE_STRING).
This was done AFTER i already put the cell's value to be the formula shown in the first thread post.
I removed that line and now the summation formula now works properly. Hopefully this helps others as well.