So, something to keep in mind is those win rates show the average. On most sites that show League stats there are two win rates displayed for each champion. One is just the baseline average for the rank level being viewed, and the other shows what experienced players and mains have on that champion. They don't typically show the main win rate on the countered page unfortunately.
I pick Garen into him, and I have also played Garen since I started back in season 1. I'm not a high elo player, but my matchups with Garen often stray from the average.
On LoLalytics Sion's listed win rate against Garen is 51.81%, and on League of Graphs Garen is listed as having a 4% lower win rate when against Sion. Op.gg shows something similar.
On League of Graphs in particular Garen has an average win rate of 49%, with Garen mains having a win rate around 56%. Now, there is a lot of room for error here, partly from me being meh with statistics, but the difference between the main win rate and average win rate gives a lot of wiggle room for people who main a champion to beat matchups that, on average, they should lose. My personal experience matches this, and I pick Garen into all sorts of nonsense that should work against me. My favorite matchups are actually against Gnar and Quinn.
Now, I'm not saying Garen is a good pick into Sion by any means. The average win rates for players who don't main Garen specifically say he's not a good pick. Plus you linked a comment from a professional player, and Garen is by no means competitive in the pro scene. I can get a way with a lot here, with Doran's Shield, Second Wind, and if the enemy has a lot of AP and Sion builds tank Spectre's Cowl, just as most Juggernauts can, but I have that extra edge of maining a champion who can do well against him.
Now that I think about it I'm not sure what I'm saying here, but hopefully you understand my personal perspective on laning against Sion.