This has been tested in 2014. A strong version of Stockfish (ELO 3200) was pitted against Nakamura (2800) with an early version of Rybka (3000) to help him. The Rybkamura team lost.

https://www.chess.com/news/view/stockfish-outlasts-nakamura-3634

So evidence is against Kasparov's claim.

This has been tested in 2014. A strong version of Stockfish (ELO 3200) was pitted against Nakamura (2800) with an early version of Rybka (3000) to help him. The Rybkamura team lost.

https://www.chess.com/news/view/stockfish-outlasts-nakamura-3634

This is evidence against Kasparov's claim.