The reason why the skoda isn't considered OP is upto 3 simple reasons; alpha, clip size, and armor.
The Skoda only has 300 alpha while the Erich has a whopping 500, that's almost double. That along with the fact that the Erich only needs to hit 2 shells as opposed to the Skoda's 3 means that the Erich on average, is much more reliable when it comes to dishing out damage.
The Erich can just poke, shoot once, reload and still get to deal 500 damage while only losing half its DPM. The Skoda on the other hand only deals 300 and loses 2/3rds of its DPM.
This ties along nicely to my next point, armor.
The Skoda might aswell be made out of cardboard with how little armor it carries, basically anything that hits it is almost certainly guaranteed to penetrate. In fact, it's lower plate is thin enough to get HE'd trough by basically any 152mm.
The Erich on the other hand, has an incredibly strong armor profile in comparison. A massive chunk of its turret sits at a crazy 240-260mm, which is enough to bounce most standard rounds of even tier 10s, and enough to make almost any tier 8 medium/light struggle even with premium. And that's not all, because the Erich also possess quite the troll hull armor. It has an upper plate that is not only 260mm, but also an autobounce on flat ground to any AP/APCR shell.
This means that the Erich doesn't even need to go hulldown to bounce shots, even just hiding the lower plate is enough to bait most shells. This is obviously a stark comparison to the Skoda, which struggles to bounce anything, even from tier 5s.
This, combined with the prior differences mentioned, results in the Erich being much, much better at trading shots, as the Erich not only needs to poke less often, penetrate less shots overall, but also have a much higher likelihood of bouncing any incoming shots alltogether.
And if that wasn't all, it also deals 100 more damage per clip, and while that doesn't sound like much, it adds up quick, even in a single battle.