How do we know it's past tense?
Can't tell out of context. Both should be correct.
As "falamos de" is correctly translated in English as either "We speak of/about" and "We spoke of/about" either translations should be accepted. There is no way to tell past tense in this case with out a prompter such as ontem.
It won't accept "of many things." Reporting it.
"Of shoes - and ships - and sealing wax-
Of cabbages- and Kings -
And why the sea is boiling hot -
And whether pigs have wings."
We talk about many things should be correct; falamos is both present and past without further context.
I submitted the answer as the present tense and it was marked as wrong