World Chess Solving Championship 2019

Hello

I am looking at www.wfcc.ch/wp-content/uploads/2019-09-21-WCSC-LTU-Vilnius-Problems.pdf
If I understand correctly, it is white to move and mate in two in the first problem.
The solution of the fist problem is Ra7 according to www.wfcc.ch/wp-content/uploads/2019-09-21-WCSC-LTU-Vilnius-Solutions.pdf
What is wrong with Nd1?

If Nd1, there's Rf3 blocking the check. Remember black pawns are moving down the board, so g4 covers f3. Ra7 is necessary to lure the rook away, so that Nd1 would be mate.