So does this verb also mean "filling IN" since that makes the most sense in this context and I'm not seeing anything else that would add the "in" part of the sentence? (when I click on ממלאים it just says fill or filling)
You could fill in for someone למלא מקום, or you could למלא טופס, like filling an empty space in a page, ect. למלא תשובה sounds peculiar to me.
למלא תשובה בטופס
It's used backwards too, using the filler as direct object