I don’t think it’s “obvious” as long as Thunder doesn’t have Instinct. While Thunder’s Instinct is good, it doesn’t really change how good some of Thunder’s normals are. As a Glacius main (for now), I’ve stopped playing ranked (due to constant DC’ers) and switched to long sets. Some of those long sets have been against very strong Thunder mains. On the surface, it’s easy to say it’s a winning match for Glacius cause of his keep out tactics, but that’s really just day/week one stuff. It’s really not that hard for Thunder to get in without Instinct, and once he’s in, it’s very very bad for Glacius.
It’s somewhat akin to Gief trying to get in for days, but once he’s in, the game is completely in his favor. What helps Thunder (and other characters) even more is the fact that you can dash before the round starts, and between rounds. Glacius cannot. He basically starts the match next to you, assuming you want to be next to him. Thus is a huge, yet under appreciated factor.
Thunder’s real power comes when people start using his normals to stuff Glacius’ limbs and understands the ranges of each Shatter.
Of course, I’m not saying this isn’t a tough fight in either direction, I’m just saying it’s not as simple as just “no Instinct for Thunder, Glacius’ favor”.