tl;dr Sinn Féin has seven MP's elected who do not take their seats. By resigning they could cause by-elections which likely would be won by anti brexit parties.

So the effect of resigning will be higher than actually taking the seat and voting against brexit?

(Assuming the seats are actually won by the anti-brexiters)

They won't take the seats because they refuse to pledge allegiance to the queen.

The article addresses this question.

As much as Sinn Fein protest against Brexit isn’t the reality that it provides the best opportunity for reunification in the near future?

I'm semi in favour of a Canada style trade deal for the mainland and NI, which voted remain, remaining in the customs union, and a border at the Irish Sea. It would be a step towards a united Ireland without changing things too much.