You can do this, but you need to do it in a couple of stages.
Let's say you've got a line item called Factor and you want the summary to be PRODUCT. You need to create an intermediate line item called Logarithm with the formula
and set its summary to Sum.
Now add another line item called Product with the formula
and set its summary to Formula.
This will multiply all the values in Factor and put the result in Product. I think this is the same approach that Jack recommended.
Re: RE: Anaplan equivalent of the "Product" Function
This solution needs more visibility. I searched for PRODUCT function equivalents or workarounds before coming up with this solution. Came to post on the community only to find it shared here >3 years ago.