After comparing your images with my own Soulsilver cartridge and case, everything seems to check out. The one thing that did stand out was that my case is only in English, yours appears to be in two languages. After looking up images for Soulsilver cases, I did find a few that were laid out like yours in two languages, but not many; it might be something specific to the seller’s area.
The actual cartridge is impeccably good. The color’s right, the E rating is proportioned properly, the transparent red when you hold it up the the light is normal, they even got the correct text code on the back and bottom of the front. I’m pretty convinced it’s legit. Just play the game and be wary of game-breaking glitches. I haven’t looked into fake DS cartridges as much as I have fake GBA carts, but fake 3rd Gen games are notorious for corrupting and deleting the player’s save file, especially after defeating the E4. The only other way I can think of to prove its legitimacy is to try migration or PokeTransfer, if you can. I don’t know if Soulsilver will allow you to migrate Pokémon from Gen 3 until you reach Fuschia City, but if you reach that point and it still won’t let you migrate, it’s fake. You should be able to transfer Pokémon from a legit Soulsilver cart to a Gen 5 game at any point in SS’s story, as long as you’ve made it to the Transfer Lab in the Gen 5 game.