After White's 22. Bc1, it seems like responding with Rd2 would keep the game alive for Black.
This is an entirely wrong assumption. Black has no counterplay whatsoever, while White holds the initiative. Being down material, Black has no resources/time to repel the coming attack, nor to organize a defense/counterplay. Because his queen is trapped he will be forced to lose even more material, which will only make things easier for White.
I assume Friedel's a more insightful player than I am. What am I missing?
[Title "Easiest way to win this position as White"]
[StartFlipped "0"]
[fen "6k1/pb2bppp/5n2/n1p1N3/3r1qP1/1PN4P/P3QP2/R1B1R1K1 b - - 0 1"]
1...Rd2 $8 2.Bxd2 Qb4 $8 3.Nd5! $18 Qa3 ( 3...Qd4 4.Nxe7+ $18 ) 4.Nxe7+ $18
With two rooks up, White has an easy task of continuing the mating attack.
That is, from an experienced player's viewpoint, why do we assume that staying alive with Rd2 is not worth the effort, and how would White be sure to win regardless?
He probably saw the line from the above diagram and decided to resign. Even though you are an inexperienced player, I doubt you would lose that position against the strongest engine on Earth, since Black has no counterplay and will be forced to exchange pieces to prolong the game after which extra rooks will decide. I will give you just one sample line that shows how I would continue as White:
[Title "Easiest way to win this position as White"]
[StartFlipped "0"]
[fen "6k1/pb2bppp/5n2/n1p1N3/3r1qP1/1PN4P/P3QP2/R1B1R1K1 b - - 0 1"]
1...Rd2 $8 2.Bxd2 Qb4 $8 3.Nd5! $18 Qa3 ( 3...Qd4 4.Nxe7+ $18 ) 4.Nxe7+ Kf8 $8 ( 4...Kh8 5.Nxf7# ) 5.Nd7+ Ke8 ( 5...Nxd7 6.Ng6+ Kg8 ( 6...fxg6 7.Qe8# ) ( 6...hxg6 7.Qe8# ) 7.Qe8+ Nf8 8.Qxf8# ) 6.Nxf6+ gxf6 ( 6...Kd8 7.Bxa5+ Qxa5 8.Qd3+ Kc7 9.Qg3+ Kb6 ( 9...Kd8 10.Qd6# ) 10.Qd6+ Kb5 ( 10...Bc6 11.Qxc6# ) 11.a4+ Kb4 ( 11...Qxa4 12.bxa4+ Kb4 ( 12...Ka5 13.Qxc5+ Ka6 14.Qb5# ) 13.Qd2+ Kc4 ( 13...Kb3 14.Re3+ Kc4 15.Qc3# ) 14.Rec1+ Kb3 15.Qc3# ) 12.Qd2+ Kxb3 13.Qd3+ Qc3 ( 13...Kb2 14.Reb1# ) ( 13...Kb4 14.Reb1# ) 14.Reb1# ) ( 6...Kf8 7.Ng6+ fxg6 ( 7...hxg6 8.Qe8# ) 8.Qe8# ) 7.Nd5+ Kd7 ( 7...Kd8 8.Qe8# ) ( 7...Kf8 8.Bh6+ Kg8 9.Qe8# ) 8.Qe8+ Kd6 9.Bf4+ Kxd5 10.Qe4#
As you can see, although the number of lines is vast, the attack is straightforward. The only "problem" I had with analyzing this for White was to find the best check with the queen so I can keep the lines forcing. I had to spend half an hour calculating all the lines but am sure a player of Nakamura's strength would find these in a matter of seconds...
The above lines very instructively illustrate the hopelessness of Black's position, so there is no need for me to add anything else. If you need further help or clarification leave a comment.
Best regards.
...Rd2 23. Bxd2 Qb4
White could grab a pawn with24. Nxf7
(if...Kxf7
then25. Qxe7+
), opening lines to the black king. It's just hopeless.22... Rd2 23.Bxd2 Qb4
, White is up two rooks for a bishop so can probably aim higher than winning a pawn... For example, 24.Nd5 wins at least a piece.