He is talking about removing a link from the chain, not taking slack from it. By removing link from the chain you can move the axle forward with the adjusters, which will shorten the wheelbase, making it turn more quickly and put a little more weight on the rear wheel, making the front end want to wheelie a bit easier as well. Like all things in suspension, and frame geometry ,there are compromises. You would give up a little bit on stability. Theoretically it would also effectively shorten the swingarm which would make the shock spring feel stiffer and decrease the amount of travel, but you would need to be very perceptive to notice these changes from removing a single link.
Do not decrease the amount of chain slack. The slack is there to accommodate for changes in chain geometry as the swingarm moves through it’s travel. If you were to take too much slack out of the chain you could damage the countershaft bearing in the transmission, wheel bearings and the chain / sprockets. You are always better to err on the loose side.