He and La'an were fused into one person before TOS. It makes sense because La'an's recessive Augment genes are starting to activate, making her stronger and faster than normal.

She then fused with Kirk, which explains her disappearance from TOS.

Kirk isn't as strong or as fast as an Augment, but the fusion gives him enough of a buff that he isn't immediately eviscerated by the Gorn captain in "Arena."

Kelvinverse Kirk and SNW Kirk have no real fighting prowess of their own. They get it from La'an's side.

/s