Akali pretty much requires snowballing her lane in order to win.
Garen doesn't even have to kill Akali. Garen just has to not die. And Garen does this very well.
Also Voyboy played this match-up earlier today and he built armguard vs. him and Garen built Hexdrinker Mercs and Garen was bullying him.
So Voyboy left to jungle and gank and Jungle Xin took over top and then they won from that.
Armguard's 45 armor and 40 ap is not going to completely flip a match up that is tilted one way because of how the kits function inherently.
Akali deals with guys like Malphite by forcing trades, then backing off an healing repeatedly until Malphite goes oom. In Garen's case, he doesn't go oom and he can also heal when they back off after a trade.
Garen passively gets a lot of Mres and has 30% reduction on all damage. That's important because one problem lanes have vs. Akali is the fact that she does magic and physical damage. So on top of him amping both he also gets 30% reduction on both when he uses W.
Akali's combo is pretty cooldown reliant. By using a pink ward during her shroud and a silence when she goes back in you stop her from dealing her burst which is also her sustain through her spellvamp. Interrupting this messes her up.
Armguard's armor does nothing vs. Garen's magic damage ult.
So no Armguard does not make the match up swing into Akali's favor, not in a vacuum.