Seems more like the lack thereof

Stardew Valley used to be published under Chucklefish. Concerned Ape separated and self published after some concerns about Chucklefish ethics.

I can see how they may have connected the Chucklefish logo from there to this game.

His best friends index starts at 0

This could be for a variety of reasons but the thing I think is most likely is that they could absorb oxygen much better avoid anaerobic respiration (low energy production)that creates lactic acid in the first place. If they are able to sustain aerobic respiration (high energy production) longer, they can have their bodily energy demands met and continue to do their activity never producing lactic acid.

The cause of delayed onset muscle soreness (DOMS) is separate from lactic acid but may be related in a person’s ability to metabolize and regenerate tissue as that guy clearly has an efficient body.