Not to seal, you would need a tanking solution for that. The hardiebacker is to provide a good medium to fix tiles to, not to completely provide a completely waterproof solution if water penetrates the tiles.
The tape is self adhesive so you don't need any filler.
If I recall, when I fixed my cement boards, the advice was to embed the fibatape in a thin film of tile adhesive over the joints.
The manufacturers recommend the use of fibatape rather than standard plastering scrim, and stainless screws or fixings, due to the alkalinity of the cement boards.
As above, cement board is not waterproof, it's water resistant.
If the boards are in a shower area, I would highly recommend using a tanking kit!
you could fill the void with the adhesive your’e using and bed some plasterers scrim into the adhesive on the joins as you go...... or not . it’s not important unless the edge of your tile lands on a join in the board.